bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
1 Animal biosynthesis of complex polyketides in a photosynthetic partnership
2 Joshua P. Torres1, Zhenjian Lin1, Jaclyn M. Winter1, Patrick J. Krug2, and Eric W. Schmidt1*
3 1Department of Medicinal Chemistry, University of Utah, Salt Lake City, UT 84112 USA 4 2Department of Biological Sciences, California State University, Los Angeles, CA 90032 USA 5 *Correspondence to: [email protected] 6
7 ABSTRACT
8 Animals are rich sources of complex polyketides, including pharmaceuticals, cosmetics, and
9 other products. Most polyketides are associated with microbial or plant metabolism1. For this
10 reason, symbiotic bacteria or dietary organisms are often the true producers of compounds found
11 in animals2,3. Although increasing evidence suggests that animals themselves make some
12 compounds4-7, the origin of most polyketides in animals remains unknown. This problem makes
13 it difficult to supply useful animal compounds as drugs and severely constrains our
14 understanding of chemical diversity and the scope of biosynthesis in nature. Here, we
15 demonstrate that animals produce microbe-like complex polyketides. We report a previously
16 undocumented but widespread branch of fatty acid synthase- (FAS)-like proteins that have been
17 retooled by evolution to synthesize complex products. One FAS-like protein uses only
18 methylmalonyl-CoA as a substrate, otherwise unknown in animal lipid metabolism, and is
19 involved in an intricate partnership between a sea slug and captured chloroplasts. The enzyme’s
20 complex, methylated polyketide product results from a metabolic interplay between algal
21 chloroplasts and animal host cells, and also likely facilitates the survival of both symbiotic
22 partners, acting as a photoprotectant for plastids and an antioxidant for the slug8-12. Thus, we find bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
23 that animals can unexpectedly synthesize a large and medically useful class of structurally
24 complex polyketides previously ascribed solely to microbes, and can use them to promote
25 symbiotic organelle maintenance. Because this represents an otherwise uncharacterized branch
26 of polyketide and fatty acid metabolism, we anticipate a large diversity of animal polyketide
27 products and enzymes awaiting discovery.
28
29 MAIN
30 Complex, branched polyketides, are canonically ascribed to microbial metabolism, yet they are
31 commonly found in animals13,14. The sacoglossan polypropionate pyrones are among the most
32 structurally complex polyketides isolated from animals (Fig. 1). Sacoglossans consume algae,
33 and some species maintain the stolen algal chloroplasts (kleptoplasts) for weeks or in exceptional
34 cases, for several months15. Feeding studies showed that fixed carbon obtained from de novo
35 photosynthesis is efficiently incorporated into polypropionates via chloroplasts16. Labeled
36 propionate is also incorporated17-20, suggesting that the mollusk compounds are made via the
37 methylmalonate pathway, which has historically been associated with bacterial metabolism24.
38 The polypropionates react via oxidative and photocyclization pathways that are likely non-
39 enzymatic to form the mature natural products8-10,16. It is thought that these reactions protect the
40 slugs from damage associated with photosynthesis, and that they may thus be necessary for a
41 photosynthetic lifestyle8-12. Thus, the polypropionates are known to originate in the mollusks and
42 to be central to mollusk biology, yet their biochemical sources were unknown.
43 bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
44 We took an unbiased approach to define the molecular source of polypropionates. To test the
45 bacterial origin hypothesis, we sequenced the metagenomes of polypropionate-containing
46 sacoglossans Plakobranchus cf. ocellatus “aff. sp. 1”21 and Elysia diomedea. We were unable to
47 identify good candidate polypropionate synthases from the bacterial metagenomes, suggesting a
48 possible origin in the animals or chloroplasts. We took advantage of recently released genomes
49 and transcriptomes of three sacoglossans11,22-24, Elysia chlorotica, E. timida, and E. cornigera, to
50 test this hypothesis. E. chlorotica contains polypropionates ent-9,10-deoxytridachione and
51 elysione25, while E. timida contains ent-9,10-deoxytridachione and several relatives (Fig. 1,
52 Extended Data Table 1)26. Both species obtain fixed carbon from chloroplasts that they maintain
53 for one (E. timida) or several (E. chlorotica) months. In contrast, no polypropionates have been
54 reported from E. cornigera, which only retains chloroplasts for a few days27.
55 Fig. 1: Biogenesis of sacoglossan polypropionates.
56 bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
57 a Previous feeding studies demonstrated that kleptoplasts in the sacoglossans fix carbon that is
58 used to make polypropionates. Propionate is also a precursor, presumably via methylmalonate.
59 A further series of hypothetical steps, including synthesis by a PKS, methylation, and
60 photochemical and oxidation reactions would lead to b the known natural polypropionates of
61 diverse structures. These are found in c sacoglossan species investigated in this work, including
62 E. chlorotica and E. timida, but not E. cornigera.
63
64 Based upon recent reports of animals that make polyene fatty acids5,6, we suspected that the
65 animals might encode modified fatty acid synthase (FAS) or PKS enzymes. Examining the
66 available E. chlorotica transcriptomes, we found a number of fragmented transcripts encoding
67 multiple type I FAS enzymes. Reassembly of those transcripts from raw read data revealed four
68 different FAS-like enzymes (Extended Data Table 2). Two of these were the cytoplasmic and
69 mitochondrial FAS enzymes of primary metabolism. The remaining two (which we named
70 EcPKS1 and EcPKS2) formed their own distinct group in a global PKS/FAS tree that included
71 bacterial, fungal, and animal PKS variants (Fig. 2A, Extended Data Fig. 1). EcPKS1, EcPKS2,
72 and cytoplasmic FAS were encoded in the animal genomes, and not in chloroplasts (Fig. 2C,
73 Extended Data Fig. 2). EcPKS1 homologs were only found in E. chlorotica and E. timida, and
74 not in E. cornigera, while the other genes were found in all three. This observation was
75 consistent with EcPKS1 being the putative polypropionate synthase.
76
77 Like other type I FAS and PKS enzymes, EcPKS1 and EcPKS2 contain multiple domains
78 responsible for product formation (Fig. 2B). Enoylreductase (ER) domains reduce double bonds bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
79 in the nascent polyketide chain. Because EcPKS1 and EcPKS2 lack functional ER domains, theyey
80 likely both synthesize polyenes (Fig. 2D)5,28. The acyltransferase (AT) domain is responsible forr
81 substrate selection, and therefore would be responsible for choosing malonyl-CoA (straight-
82 chain lipids) or methylmalonyl-CoA (methyl-branched lipids)29. In bacteria, amino acid residuess
83 responsible for AT selectivity are well understood30. Since no sequenced animal FASs/PKSs
84 prefer methylmalonyl-CoA, there is no model for AT selectivity in animals. Gratifyingly,
85 EcPKS1, EcPKS2, and several relatives, contained a conserved GHSXGE sequence motif found
86 in bacterial ATs that is known to be important in substrate selection. In bacteria, when X is
87 methionine, the substrate is methylmalonyl-CoA. In EcPKS1, a GHSMGE sequence was
88 observed, whereas that of EcPKS2 was GHSLGE (Fig. 2E). This implied that EcPKS1 might usese
89 methylmalonyl-CoA, while EcPKS2 should use malonyl-CoA. Thus, EcPKS1 had all of the
90 sequence and expression features consistent with the polypropionate synthase.
91 Fig. 2: Sacoglossan PKSs, a new class of FAS-like PKS enyzmes.
92 bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
93 a Sacoglossan PKSs represent a group separate from the known FAS and PKS proteins from
94 bacteria, fungi and animals. Numbers indicate consensus support in percent. Outgroup DpsA is
95 a type II PKS sequence from bacteria, while the remainder are type I enzymes. b The domain
96 architecture of the sacoglossan PKSs, EcPKS1 and EcPKS2 includes ketosynthase (KS),
97 acyltransferase (AT), methyltransferase (MT), dehydratase (DH), enoylreductase (ER),
98 ketoreductase (KR) and acyl carrier protein (ACP). This domain architecture is identical to the
99 sacoglossan FAS, EcFAS except for the absence of a C-terminal thioesterase domain. c EcPKS1 is
100 encoded in the genome of E. chlorotica. d The acyltransferase (AT) domain sequence indicates
101 loading preference for methymalonate while the e enoylreductase (ER) domain lacks key
102 NADPH binding site residues making this function inactive.
103
104 EcPKS1 gene was synthesized and expressed solubly in Saccharomyces cerevisiae BJ5464
105 harboring the npgA phosphopantetheinyl transferase gene (Extended Data Fig. 3)31. Incubation of
106 the purified enzyme with methylmalonyl-CoA and NADPH led to a series of new peaks in the
107 chromatographic trace (Fig. 3A, Extended Data Fig. 4). The major peak had a UV spectrum and
108 mass consistent with a triene pyrone that is three carbons smaller than the major natural products
109 of E. chlorotica (Fig. 3B). A second, minor tetraene pyrone product had mass and UV features
110 consistent with the putative precursor of tridiachiones and related natural products17.
111
112 Like other unmethylated, polyunsaturated pyrones, the enzymatic reaction products were
113 challenging to isolate and characterize because of chemical instability. To gain further evidence
114 for the presence of the pyrone structure, we hydrogenated and derivatized the enzymatic reaction bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
115 mixture, and then analyzed the products by gas chromatography-mass spectrometry (GCMS).
116 This method was chosen because of its predictive value: hydrogenation of the triene should
117 produce four separable stereoisomers, with predictable fragmentation serving to localize methyl
118 groups. Hydrogenation gave four isomers with nearly identical fragmentation patterns around
119 some of the methyl groups, supporting the structural assignment (Extended Data Fig. 5).
120
121 Although some animal FAS/PKS enzymes accept methylmalonyl-CoA in vitro, malonyl-CoA is
122 preferred32-34. We sought to determine whether EcPKS1 specifically incorporates
123 methylmalonate, or whether it also uses malonyl-CoA like other enzymes. We incubated
124 EcPKS1 with malonyl-, methylmalonyl-, acetyl-, and propionyl-CoA under various conditions
125 (Fig. 3D, Extended Data Fig. 6). These experiments demonstrate that EcPKS1 uses only
126 methylmalonyl-CoA, and not other substrates. Further, our data suggest that other CoA esters
127 compete with methylmalonyl-CoA but are not incorporated into any products, indicating
128 stringent selectivity for methylmalonate. S-adenosylmethionine (SAM) is the source of methyl
129 groups in many branched polyketides28. We co-incubated SAM with malonyl-CoA and
130 methylmalonyl-CoA in the presence of EcPKS1, showing that SAM is not involved in the
131 biosynthesis (Extended Data Fig. 6). Thus, EcPKS1 is the first characterized methylmalonate-
132 specific FAS/PKS from animals.
133
134 These data revealed that EcPKS1 is a mollusk polypropionate synthase. In combination with
135 previous experimental evidence, a biosynthetic pathway from CO2 to complex polyketides can
136 be defined. Symbiotic chloroplasts fix CO2, which is then converted into methylmalonyl-CoA.
137 The animal enzyme EcPKS1 synthesizes the tetraene pyrone, which would be converted to bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
138 tridachiones and related natural products by methylation, photorearrangement, and oxidation.
139 These later events may be important in photosynthesis and are strongly supported by previous
140 synthetic chemistry experiments in which the γ-methylated tetraene pyrone is used as starting
141 material to synthesize the natural products (Extended Data Fig. 7)35-37.
142
143 The close connection between chloroplasts and polyene pyrones extends beyond biosynthesis. It
144 has been widely observed that polyene pyrones are found in sacoglossans that engage in long-
145 term photosynthesis, while those lacking polyene pyrones also derive little or no nutrition from
146 photosynthesis (Extended Data Table 1). We found that, in E. diomedea, polyene pyrones are not
147 found in developing embryos, which lack chloroplasts (Extended Data Fig. 8). Moreover, when
148 examining transcriptomes of E. chlorotica at various growth stages, we found that EcPKS1 is
149 highly expressed as the animals metamorphose from the planktonic larval to the benthic juvenile
150 slug stage, just before chloroplasts are acquired (Extended Data Fig. 9). The expression
151 continues well into the stage when the animal maintains stable chloroplasts. In E. timida,
152 expression of the EcPKS1 ortholog EtPKS1 is downregulated after prolonged starvation, when
153 sequestered chloroplasts have degraded.
154
155 To examine the association of EcPKS1 with photosynthetic ability further, we reinvestigated the
156 P. cf. ocellatus and E. diomedea metagenomes with the hypothesis that EcPKS1 homologs would
157 be found in each, as these species are also capable of long-term chloroplast retention. Indeed,
158 homologs of EcPKS1, EcPKS2, and cytoplasmic FAS were discovered (Extended Data Fig. 10).
159 These were mollusk-encoded, as they came from intron-bearing contigs similar to those
160 encoding their relatives in E. chlorotica. Our data thus strongly support previous suggestions that bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
161 pyrones are critical for maintaining long-term photosynthetic activity in sacoglossans by servingg
162 antioxidant and photoprotective roles.
163 Fig: 3 EcPKS1 is a methylmalonate-specific animal FAS/PKS that synthesizes the tridachione
164 precursor.
165
166 a Incubation of EcPKS1 with methylmalonyl-CoA and NADPH resulted in the synthesis of
167 polyene 1 and tridachione precursor 2. b UV spectra of the synthesized products showing
168 features of polyene propionates consistent with the structural assignment. c Incubation with
169 other acyl-CoA substrates (acetyl-CoA: a-CoA, malonyl-CoA: m-CoA, propionyl-CoA: p-CoA,
170 methylmalonyl-CoA: mM-CoA ) with NADPH did not yield new products. d Using propionyl-CoA bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
171 as the loading molecule inhibited the reaction, as observed in decreased yields. e Using
172 different loading and extender units other than methylmalonate did not produce any products.
173 Graphs shown in a and c-e are HPLC-DAD experiments, where the x-axis gives the elution time
174 in minutes and the y-axis is absorbance units at λ = 325 nm. The plots shown in b are UV
175 spectra with the y-axis showing absorbance and the x-axis showing wavelength in nanometers.
176
177 Polypropionates in marine mollusks are abundant and structurally diverse13. Aside from those
178 described in this study, no other sequenced mollusks are known to contain polypropionates.
179 Examining non-propionate mollusks in GenBank, we found that octopus, sea hares, clams, and
180 oysters contain many FAS-type PKSs that have not been previously noted. However, none of
181 these enzymes falls within the sacoglossan polypropionate group. So far two sequenced
182 transcriptomes encode the methylmalonate-specific motif, from the hemichordate Saccoglossus
183 kowalevskii and the beetle Leptinotarsa decemlineata. These data imply that the ability to
184 synthesize complex polyketides is a ubiquitous feature among mollusks, and it may therefore
185 play an important and unrecognized biological role in the animals.
186
187 Using these newly identified sequences, we generated a taxonomically broader phylogenetic tree
188 based upon the ketosynthase domains of FAS/PKS genes (Extended Data Fig. 10). Many of the
189 mollusk PKSs, as well as several from other phyla, fall in a single large group that includes
190 animal FAS. A second large group contains all of the characterized type I PKS genes from
191 bacteria, animals, and fungi. EcPKS1 is the only characterized PKS within the large FAS-like
192 PKS cluster, representing a new biochemical frontier for PKS research. bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
193
194 A remaining mystery involves the in vitro product selectivity observed in our study. The major
195 enzymatic product incorporated six units of methylmalonate, while the minor product
196 incorporated seven. While many mollusk polypropionates contain five or seven equivalents of
197 methylmalonate, none of the reported compounds contains six. We are therefore still missing a
198 chain length-determining factor. EcPKS1 lacks a thioesterase (TE) domain, which is often
199 important for chain length determination38. We attempted to use base to improve offloading of
200 the product39, but were unsuccessful, indicating that an as-yet unknown factor governs chain
201 length (Extended Data Fig. 6). We speculate that this factor is likely to be the unknown
202 methyltransferase involved in the next step of the biosynthetic pathway.
203
204 In summary, EcPKS1 represents the first example of complex polyketide biosynthesis in animal
205 metabolism. We use biochemical evidence to demonstrate that structurally elaborate polyketides,
206 a family previously associated with microbial metabolism, are animal products as well. The
207 biosynthesis is part of an elaborate molecular dance in which kleptoplasts from algae fix CO2.
208 Fixed carbon is transformed into methylmalonyl-CoA and modified by the mollusk EcPKS1
209 enzyme to synthesize UV- and oxidation-blocking pyrones that protect the mollusk and its
210 chloroplasts from photosynthetic damage.
211
212 Methods
213 Genomic DNA extraction, sequencing and assembly bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
214 Single live specimens of Plakobranchus cf. ocellatus “aff. sp. 1” sensu (collected by SCUBA in
215 the Solomon Islands, June 22, 2006; 9° 1.02’S 160° 13.06’ E) and Elysia diomedea (purchased
216 from www.Liveaquaria.com, Rhinelander, WI, USA) were used in this work. Tissue sections
217 each approximately 0.5 cm3 were frozen using liquid nitrogen and ground using a sterile mortar
218 and pestle. Ground tissue was resuspended in lysis buffer (2 mL; 100 mM NaCl, 10 mM EDTA,
219 1.0 % SDS, 50 mM Tris-HCl at pH 7.5) followed by addition of proteinase K (25 µL; 20
220 mg/mL) and incubated in a water bath at 55 °C until lysis was complete (clear solution). The
221 solution was removed from the water bath and RNAse (2 µL; 20 μg/mL) was added. After 15
222 minutes of incubation at 37 °C, saturated KCl (200 µL) was added to the solution. The mixture
223 was incubated in an ice bath for 10 minutes and then centrifugated at 3,739 x g at 4 °C.
224 Phenol:chloroform extraction of the supernatant40 led to purified DNA. Sequencing was
225 performed at the Huntsman Cancer Institute’s High Throughput Genomics Center, University of
226 Utah. Sacoglossan genomes were sequenced using Illumina HiSeq 2000 sequencer with 350 bp
227 inserts and 125 bp paired-end runs. Raw reads were merged using BBMerge41. Non-merged
228 reads were filtered and trimmed using Sickle with the parameters (pe sanger -q 30 –l 40)42. The
229 trimmed and merged FASTQ files were assembled using metaSPAdes43 with standard
230 parameters in the Center for High Performance Computing at the University of Utah.
231
232 Identification of polypropionate PKS genes in published sacoglossan transcriptomes
233 Sacoglossan transcriptome reads were acquired in previous work by other groups 11,23 from
234 19 specimens of Elysia chlorotica, Elysia timida, and Elysia cornigera (Extended Data Table
235 3). We downloaded the reads from the NCBI Sequence Read Archive (SRA) and assembled
236 them using IDBA-UD44. The assembled transcriptomes were queried by Basic Local bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
237 Alignment Search Tool (BLAST) using the Homo sapiens FASN gene (NCBI Accession:
238 NP_0045095.4) involved in human fatty acid metabolism. Putative full-length, assembled
239 FASN-related transcripts were translated using ExPASy translation tool. FASN and related
240 sacoglossan transcripts consist of multiple domains, which were annotated using
241 Conserved Domain Database (NCBI)45 and Pfam 32.0 (EMBL)46. Geneious 10.2.2 software
242 package was used to perform alignments and tree building. Sequence alignments were
243 made using Multiple Sequence Comparison by Log Expectation47, trimmed and realigned.
244 Phlyogenetic trees were constructed using the Unweighted Pair Group Method with the
245 Jukes-Cantor model48.
246
247 Construction of EcPKS1 expression plasmid
248 The plasmid was constructed by homologous recombination in yeast. The predicted EcPKS1
249 gene was synthesized in three fragments containing 70-bp overlapping regions (Genewiz). The
250 fragments were designed so that their ends overlapped with the promoter and terminator sites of
251 yeast expression plasmid39. The expression vector was linearized using NdeI and PdeII (New
252 England Biolabs) following the manufacturer’s protocol, and the EcPKS1 fragments and
253 linearized vector were cotransformed into Saccharomyces cerevisiae BJ5464 using the S.C.
254 EasyComp Transformation Kit (Sigma). Transformants were plated onto synthetic complete
255 media and selected by transformation to uracil prototrophy. Yeast colonies were combined, and
256 plasmids were isolated using the QIAprep Spin Miniprep Kit (QIAGEN). Plasmids were
257 amplified by transformation into chemically competent Escherichia coli DH5α (New England
258 Biolabs), which was plated onto LB agar (10 g/L tryptone, 5 g/L yeast extract, 10 g/L NaCl, agar
259 20 g/L) with ampicillin (50 μg/mL) and incubated for 24 hours at 37 oC. Individual colonies were bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
260 picked and inoculated in to 5 mL LB broth with ampicillin (50 μg/mL) and incubated at 30 °C
261 with overnight shaking. Plasmids were obtained using the QIAPREP Spin Miniprep Kit
262 (QIAGEN) and screened by digestion with BamHI (New England Biolabs) followed by agarose
263 gel electrophoresis. Plasmid pJTPKS1 containing the expected insert was verified by Sanger
264 sequencing.
265
266 Overexpression and purification of EcPKS1
267 Plasmid pJTPKS1 was transformed into Saccharomyces cerevisiae BJ5464-NpgA (MATα ura3-
268 52 trp1 leu2-Δ1 his3Δ200 pep::HIS3 prb1d1.6R can1 GAL), which was selected on uracil-
269 deficient agar (1.39 g/L Yeast Synthetic Drop-out Media Supplements without uracil (Sigma-
270 Aldrich), 6.7 g/L Yeast Nitrogen Base (Sigma-Aldrich), 40 mL/L 50% glucose solution, agar 20
271 g/L) and incubated at 30 °C for 48 hours. A single colony of BJ5464-NpgA-EcPKS1 was used to
272 inoculate uracil-deficient broth (5 mL; 1.39 g/L Yeast Synthetic Drop-out Media Supplements
273 without uracil (Sigma-Aldrich), 6.7 g/L Yeast Nitrogen Base (Sigma-Aldrich), 40 mL/L 50%
274 glucose solution) and incubated at 30 oC with shaking at 150 rpm. After 24 hours, 1 mL of the
275 culture was used to inoculate 6 x 1 L yeast peptone dextrose broth (10 g/L yeast extract, 20 g/L
276 peptone, 20 g/L glucose). The culture was grown at 30 oC under constant shaking at 180 rpm.
277 After 72 hours, the cells were pelleted at 3,739 x g for 20 minutes at 4 oC. The cell pellets were
278 resuspended in lysis buffer (50 mM NaH2PO4, 150 mM NaCl, 10 mM imidazole, pH 8.0) and
279 sonicated on ice at 1-minute intervals until homogenous. The resulting mixture was centrifugated
280 at 28,928 x g for 40 minutes at 4 oC to separate the supernatant from the cell debris. The
281 supernatant was filtered using a 0.45-micron PVDF syringe filter (Millex-HV, Sigma) before
282 adding Ni-NTA and incubating the mixture at 4 oC for 12 hours. Soluble EcPKS1 in the lysis bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
283 buffer was loaded onto Ni-NTA resin in a gravity column. The resin was washed with 25 mL
284 each of 20 mM and 50 mM imidazole in 50 mM Tric-HCl buffer (500 mM NaCl, pH 8.0). The
285 protein was eluted using 3 x 5 mL 250 mM imidazole in 50 mM Tris-HCl buffer pH 8.0. 15 mL
286 of eluted EcPKS1 was concentrated to 200 μL, buffer exchanged (15 mL of 50 mM Tris-HCl, 2
287 mM EDTA, 5 mM DTT, pH 8.0) and further concentrated to 200 μL final volume using Amicon
288 Ultra 100 MWCO centrifugal filters (EMD Millipore). Protein concentration was calculated to
289 be 6.2 mg/L using the Bradford assay with bovine serum albumin (New England Biolabs) as a
290 standard.
291
292 in vitro characterization of EcPKS1
293 Optimization of the enzymatic reaction was done using 100-µL enzyme reaction mixtures
294 consisting of 10 μM EcPKS1, 2 mM NADPH, 1 mM DTT and 2 mM methylmalonyl-CoA
295 buffered using 100 mM sodium phosphate (over a range of pH 6.5-8.0) and incubated at room
296 temperature under dark conditions. After 16 hours of incubation, reaction products were
297 extracted twice with ethyl acetate (200 μL). The organic layer was dried using a speed vacuum
298 concentrator and reconstituted with methanol and analyzed by HPLC using a Hitachi Primade
299 HPLC system equipped with a PDA detector and autoinjector. Aliquots (20 μL) of the sample
300 were injected onto a reversed-phase C18 analytical column (Luna C18 4.6 x 100mm,
301 Phenomenex) with a gradient starting at 20% mobile phase B (H2O + 0.1 % TFA: MeCN) for 5
302 minutes and increased until 100 % mobile phase B for 20 minutes at a rate of 0.7 mL/min. The
303 optimum working pH for EcPKS1 was determined to be 7.0 (Extended Data Fig. 6). HRMS
304 analysis was performed at the University of Utah’s Mass Spectrometry and Proteomics Core
305 using an Agilent 1290 equipped with an Agilent 1290 FlexCube and Agilent 6350 Accurate bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
306 Mass Q-TOF dual ESI mass spectrometer. Sample (10 μL) was injected onto a UPHLC C18
307 column (2.1 x 150 mm Eclipse Poroshell, 1.9 μ ID) set at 30 °C. Elution was done using solvents
308 containing 10 mM ammonium formate (AF) and 0.1% formic acid (FA). Mobile phase A
309 contained water with AF and FA, while mobile phase B contained 95% acetonitrile (aqueous)
310 with AF and FA. The gradient started at 10% mobile phase B for 1 minute, increasing to 100% B
311 over 10 minutes. The flow rate was 0.5 mL/min. Electrospray ionization was done in positive
312 mode using nitrogen as sheath gas at 350 °C flowing at 12 L/min. Capillary and nozzle voltages
313 were set to 3500 V and 1000 V, respectively.
314
315 EcPKS1 product offloading assay
316 Six 100-µL reaction mixtures containing 2 mM methylmalonyl-CoA, 1 mM DTT, 2 mM
317 NADPH and 10 μM EcPKS1 in 100 mM phosphate buffered at pH 7.0 were incubated in the
318 dark at room temperature for 16 hours. After incubation, 1 M NaOH (20 µL) was added to three
319 of the reaction mixtures, which were then incubated at 65 °C for 10 minutes. Reaction products
320 were extracted into ethyl acetate extraction analyzed by HPLC. For comparison, three control
321 reaction mixtures were made using the same method, but not subjected to NaOH treatment
322 (Extended Data Fig. 6).
323
324 Methytransferase domain activity assay
325 Reaction mixtures (100-µL) containing 2 mM malonyl-CoA, 1 mM DTT, 2 mM NADPH, 5 mM
326 S-adenosyl-L-methionine (SAM), 1mM MgCl2 and 10 μM EcPKS1 in 100 mM phosphate buffer
327 (pH 7.0) were incubated in the dark at room temperature. Two parallel enzymatic reaction bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
328 mixtures containing additional 2 mM methlymalonly-CoA and one without SAM and MgCl2 but
329 with 2 mM methylmalonyl-CoA were carried out for comparison. After 16 hours of incubation,
330 reaction products were extracted with two volumes of ethyl acetate method and analyzed using
331 HPLC (Extended Data Fig. 6).
332
333 Acyl substrate specificity assays
334 Acyl-CoA substrates malonyl-Co A, acetyl-CoA, propionyl-CoA, and methylmalonyl-CoA were
335 purchased from CoALA Biosciences. Enzyme reaction mixtures (100 μL) consisted of enzyme
336 EcPKS1 (10 μM), sodium phosphate pH 7.0 (100 mM), NADPH (2 mM), DTT (1 mM) and
337 acyl-CoA substrates (2 mM). When multiple acyl-CoA substrates were used in a single reaction,
338 they were used at 2 mM each. Reaction mixtures were incubated at room temperature for 16
339 hours and protected from light to avoid photochemical rearrangements (Extended Data Fig. 6).
340
341 Chemical characterization of enzyme reaction products
342 To generate EcPKS1 products for chemical characterization, fifty enzyme reaction mixtures
343 (100-µL) containing 2 mM methylmalonyl-CoA, 1 mM DTT, 2 mM NADPH and 10 µM
344 EcPKS1 in 100 mM phosphate buffer (pH 7.0) were incubated at room temperature for 48 hours
345 with addition of 200 mM NADPH (2 µL) to replenish NADPH after the 24th hour. Enzyme
346 reaction mixtures were pooled and loaded into C18 resin packed in cotton-plugged borosilicate
347 glass Pasteur pipets. The resin was washed twice with water (1 mL) followed by 20%
348 acetonitrile:water (1 mL) before elution with acetonitrile (3 mL). The eluant was dried using a
349 speed vacuum concentrator and reconstituted in methanol 400 µL. Pd/C (0.5 mg) was added to bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
350 the solution and stirred for 16 hours under a H2 atmosphere. The mixture was then filtered
351 through Celite to remove Pd/C and dried a using speed vacuum concentrator. GC-MS/MS
352 analysis was performed at the University of Utah’s Metabolomics Core.
353
354 GC-MS analysis used an Agilent 7200 GC-QTOF and an Agilent 7693A automatic liquid
355 sampler. Dried samples were suspended in dry pyridine (100 µL; EMD Millipore) containing O-
356 methoxylamine hydrochloride (40 mg/mL; MP Bio) and incubated for one hour at 37 °C in a
357 sand bath. An aliquot (50 µL) of the solution was added to autosampler vials. N-Methyl-N-
358 trimethylsilyltrifluoracetamide containing 1% chlorotrimethylsilane (60 µL; Thermo) was added
359 automatically via the autosampler and incubated for 30 minutes at 37 °C. After incubation,
360 samples were vortexed and the prepared sample (1 µL) was injected into the gas chromatograph
361 inlet in the split mode with the inlet temperature held at 250 °C. A 1:1 split ratio was used for
362 analysis. The gas chromatograph had an initial temperature of 60°C for one minute followed by a
363 10 °C/min ramp to 325 °C and a hold time of 10 minutes. A 30-meter Agilent Zorbax DB-5MS
364 with 10 m Duraguard capillary column was employed for chromatographic separation. Helium
365 was used as the carrier gas at a rate of 1 mL/min.
366
+ 367 Polyene (1): UV (MeCN) λmax 279, 318 nm. HRESIMS m/z 289.1814 [M+H] (calcd for
368 C18H24O3, 289.1798). GCMS/MS of 1c: Parent mass m/z 366.2590 (calcd for C21H38O3Si,
369 366.2548), Fragments: m/z 73.0475 (calcd for C3H9OSi, 73.0474), m/z 155.0873 (calcd for
370 C11H23, 155.1800), m/z 211.1141 (calcd for C20H35O3Si, 211.0790), m/z 253.1239 (calcd for
371 C13H21O3Si, 253.1260), m/z 351.2322 (calcd for C20H35O3Si, 351.2355). bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
372
+ 373 Tridachione precursor (2): UV (MeCN) λmax 260, 355 nm. HRESIMS m/z 329.2120 [M+H]
374 (calcd for C21H28O3 , 329.2111).
375
376 Chemical analysis of E. diomedea
377 Chemical extracts were prepared by homogenizing egg ribbons and tissue sections
378 (approximately 10 cm3) of E. diomedea with acetone (5 mL). The homogenate was centrifuged at
379 3,739 x g for 20 minutes at 4 oC and the supernatant recovered by aspiration. The acetone extract
380 was dried using a rotovap and resuspended in methanol. Extracts were analyzed using a HPLC
381 using a Hitachi Primade HPLC system equipped with a PDA detector and autoinjector. Aliquots
382 (20 µL) were loaded onto a C18 analytical column (Luna C18 4.6 x 100mm, Phenomenex) with
383 a gradient starting at 30% mobile phase B (H2O + 0.1 % TFA: MeCN) and 70% mobile phase A
384 (H2O) for 5 minutes and increased until 100 % mobile phase B for 25 minutes at a rate of 1.0
385 mL/min. Purification of the major polypropionate in E. diomedea tissue extract was performed
386 using a semi-preparative HPLC column (Luna 5u C18 250 x 10 mm) at a flowrate of 2.5
387 mL/min. 1H NMR data were obtained using a Varian 500 (1H 500 MHz) NMR spectrometer
388 equipped with a 3mm Nalorac MDBG probe, utilizing residual CDCl3 signal for referencing.
389 Natural products, including tridachione, were identified by comparing data to those reported in
390 the literature. A similar procedure was used to isolate and identify products from P. cf. ocellatus.
391
392 Discovery of EcFAS, EcPKS1, and EcPKS2 analogs in P. cf. ocellatus and E. diomedea bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
393 Gene prediction was done using AUGUSTUS49. Full-length EcPKS1, EcPKS2, and EcFAS genes
394 were used as queries to search for PKS gene-containing contigs by tblastn search (E-value = 1.0
395 x e-10) in P. cf. ocellatus and E. diomedea genomic assembly. Contig hits were then submitted to
396 AUGUSTUS to predict genes based on parameters built using the reported E. chlorotica genome
397 assembly and protein sequences50. The resulting predicted gene fragments were queried against
398 EcPKS1, EcPKS2 and EcFAS using blastp search (E-value = 1.0 x e-10) and were ranked
399 according to bitscore and E-value to determine the best hit. The best hits for EcPKS1, EcPKS2
400 and EcFAS were assigned to each query gene then aligned and assembled to create the predicted
401 PKS and FAS orthologs in P. cf. ocellatus aff. sp. 1 and E. diomedea (PoPKS1, PoPKS2, PoFAS
402 for P. cf. ocellatus aff. sp. 1 and EdPKS1, EdPKS2 and EdFAS for E. diomedea).
403
404 EcPKS1 gene expression analysis
405 Raw reads were trimmed by Sickle with parameters (pe sanger –q 30 –l 40). Trimmed reads were
406 pooled by species, and de novo assembly for each species was done (EC and ET) using
407 maSPAdes with default parameters. Trimmed raw reads of each specimen were multimapped to
408 the reference transcriptome assembly using Salmon51 with parameters (salmon index –t –i index
409 –k 31; salmon quant --index –valudateMappings –libType A –dumpEq –r). The multitude of
410 contigs produced by assembly were hierarchically clustered, and cluster count summarization
411 was performed using Corset 1.0452 according to shared read information with parameters (-f true
412 –g –n – salmon+eg_classes). Transcript abundance of clusters (EcPKS1 gene) was estimated
413 using EdgeR53 by normalized counts per million.
414 bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
415 Data and materials availability
416 Plasmids are available from the corresponding author with a standard MTA. All sequences
417 described in this study have been deposited in GenBank (See Extended Data Table 3 for
418 accession numbers).
419
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590 Acknowledgements
591 We thank James Cox and John Alan Maschek from the University of Utah Metabolomics Core
592 Facility, which is supported by 1 S10 OD016232-01, 1 S10 OD021505-01 and 1 U54
593 DK110858-01. Yi Tang kindly provided S. cerevisiae BJ5464-NpgA. Paul Jones from Wake
594 Forest University and John Moses from LaTrobe University provided helpful discussion and
595 advice concerning the chemistry of mollusk pyrones. We are grateful to the people and nation of bioRxiv preprint doi: https://doi.org/10.1101/764225; this version posted September 10, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license.
596 Solomon Islands for permission to perform the study on P. ocellatus following Nagoya Protocol
597 standards. J.P.T., Z.L., and E.W.S. are supported by NIH R35GM122521 and U01TW008163.
598
599 Author information
600 Affiliations
601 Department of Medicinal Chemistry, University of Utah, Salt Lake City, UT 84112 USA
602 Joshua P. Torres, Zhenjian Lin, Jaclyn M. Winter & Eric W. Schmidt
603 Department of Biological Sciences, California State University, Los Angeles, CA 90032 USA
604 Patrick J. Krug
605 Contributions
606 J.P.T. and E.W.S. conceived of the project. J.P.T. performed experiments and bioinformatics to
607 discover EcPKS1. Z.L. performed bioinformatics. J.M.W. provided assistance and guidance
608 concerning yeast expression and biochemical characterization experiments. P.J.K. helped to
609 interpret the results and helped with mollusk taxonomy. E.W.S. and J.P.T. wrote the manuscript
610 with input from all authors.
611
612 Corresponding author
613 Correspondence to Eric W. Schmidt ([email protected]).
614 Competing interests
615 The authors declare no competing interests.