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Characterization of a Gene Controlling Heterocyst Differentiation in the Cyanobacterium Anabaena 7120

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Characterization of a Gene Controlling Heterocyst Differentiation in the Cyanobacterium Anabaena 7120 Downloaded from genesdev.cshlp.org on September 25, 2021 - Published by Cold Spring Harbor Laboratory Press Characterization of a gene controlling heterocyst differentiation in the cyanobacterium Anabaena 7120 William J. Buikema and Robert Haselkorn Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, Illinois 60637 USA Anabaena 7120 mutant 216 fails to differentiate heterocysts. We previously identified a 2.4-kb wild-type DNA fragment able to complement this mutant. We show here that the sequence of this fragment contains a single open reading frame [hetR), encoding a 299-amino-acid protein. Conjugation of deletion subclones of this fragment into strain 216 showed that the /ieti?-coding region is both necessary and sufficient for complementation of the Het~ phenotype. The mutation in 216 is located at nucleotide 535 in the betR gene, converting a serine at position 179 in the wild-type protein to an asparagine in the mutant. Interruption of the hetR gene in wild-type cells results in a mutant phenotype identical to that of 216. Both 216 and wild-type cells containing wild-type hetR on a plasmid display increased frequency of heterocysts, even on media containing fixed nitrogen. These results suggest that hetR encodes a product that is not only essential for but also controls heterocyst development. This putative regulatory protein lacks known structural motifs characteristic of transcription factors and probably acts at a level one or more steps removed from its target genes. [Key Words: Cyanobacteria; heterocyst; nitrogen fixation; Anabaena 7120; regulation; development] Received December 7, 1990; accepted December 28, 1990. Cyanobacteria are a diverse family of prokaryotes that Wolk 1973; Haselkorn 1978; Stewart 1980; Wolk 1982; carry out oxygenic photosynthesis similar to green Carr 1983; Bohme and Haselkorn 1988). plants. Filamentous cyanobacteria grow in long chains of Changes in intercellular communication are also ap­ identical vegetative cells when combined nitrogen is parent as a consequence of heterocyst differentiation. In available. When fixed nitrogen is limiting, many strains differentiated filaments, carbon compounds from neigh­ of filamentous cyanobacteria respond by activating boring vegetative cells are transported into heterocysts genes encoding the subunits of nitrogenase. In hetero- (Wolk 1968). Glutamine, carrying newly fixed nitrogen cystous cyanobacteria such as Anabaena 7120, the fila­ in its amide group, is transported from heterocysts to ments also undergo patterned cellular differentiation in vegetative cells (Meeks et al. 1978). In Anabaena 7120, response to nitrogen limitation. Terminally differenti­ heterocysts are spaced approximately 10-20 cells apart, ated cells called heterocysts develop at intervals along depending on growth conditions. Heterocysts inhibit the the filament, providing the sites at which nitrogen fixa­ differentiation of adjacent vegetative cells, and as the tion occurs. Heterocysts differ from vegetative cells in a vegetative interval lengthens new heterocysts form mid­ number of functionally important ways. To limit oxygen way between existing ones (Wilcox et al. 1973a). Thus, entry and protect nitrogenase from inactivation, devel­ the heterocyst spacing pattern is maintained during oping heterocysts (proheterocysts) form a new external growth on Nj as nitrogen source. Transport and metab­ cell envelope containing inner glycolipid and outer olism of a specific nitrogenous compound such as glu­ polysaccharide layers. Other structural changes include tamine has been proposed as the source of this pattern the reorganization of photosynthetic membranes and the (Haselkorn 1978), but this is unlikely in light of the nor­ formation of specialized junctions with neighboring veg­ mal heterocyst spacing patterns that are seen in most etative cells. Many biochemical changes occur in the mutants defective in nitrogen fixation (W.J. Buikema and heterocyst, including the inactivation of ribulose bis- R. Haselkorn, unpubl.; Wilcox et al. 1975). phosphate carboxylase and the oxygen-evolving photo- The formation and maintenance of heterocysts is system II (PSII) photochemical reaction center complex, strongly influenced by neighboring vegetative cells and and the induction of genes involved in nitrogen fixation growth conditions. A classic study by Wilcox et al. using and its support, such as the oxidative pentose pathway filament breakage showed that proheterocysts revert at and a heterocyst-specific ferredoxin (Winkenbach and higher frequencies and at later stages when fewer adja- GENES & DEVELOPMENT 5:321-330 © 1991 by Cold Spring Harbor Laboratory Press ISSN 0890-9369/91 $1.00 321 Downloaded from genesdev.cshlp.org on September 25, 2021 - Published by Cold Spring Harbor Laboratory Press Buikema and Haselkotn cent vegetative cells are present to support them (Wilcox sequence to GenPept (release 64), the translated protein et al. 1973b). This is not surprising considering that reading frames from GenBank. The expected gene prod­ vegetative cells act not only as a sink for fixed nitrogen uct shows no evidence of extended hydrophobic regions from the heterocysts, but also as the source of fixed car­ indicative of a signal sequence or membrane localiza­ bon to the heterocysts. This carbon serves not only as an tion. The protein contains over 30% charged residues (92 energy and reductant source for nitrogen fixation, but as of 299), although the net charge is only -6, giving an a substrate for the addition of ammonia from the nitro- expected pi of 6.37. The codon usage of hetR when com­ genase enzyme complex. pared to highly expressed Anabaena 7120 genes suggests Earlier we described the isolation and characterization that it is expressed at a low to moderate level (data not of chemically induced Fix" mutants of Anabaena 7120, shown). Secondary structure prediction algorithms including the complementation of a strain defective in (Chou and Fasman 1978; Garnier et al. 1978) suggest a an early step of heterocyst differentiation (Buikema and predominance of a-helical over (i-sheet structures. The Haselkom 1991). Here vi^e describe the characterization hetR peptide sequence does not contain significant sim­ of the hetR gene of Anabaena 7120 that is required for ilarities to common DNA-binding motifs like helix- this early step and is capable of inducing multiple adja­ turn-helix, zinc finger, or helix-loop-helix, as docu­ cent heterocysts when present on a plasmid in wild-type mented in the PROSITE database (Bairoch 1989). cells. Complementation of mutant 216 using deletion Results subclones Sequence of the hetR gene We sought to determine the minimum genetic require­ ment for complementation of strain 216 by using some Earlier we had identified and cloned a 2.4-kb partial of the deletions constructed for DNA sequencing. Se­ SfluSAI fragment of wild-type DNA able to complement lected deletions were subcloned into the polylinker of the heterocyst-deficient mutant 216 (Buikema and Ha- the shuttle vector pCCBllO (described in Materials and selkorn 1991). A physical map of this fragment is shown methods), and the resultant plasmids were transferred in Figure 1. The DNA sequence of the entire 2.4-kb frag­ via conjugation into strain 216. Figure 1 shows the dele­ ment was determined and is shown in Figure 2. The se­ tions that were used in this analysis and indicates those quence contains one complete open reading frame (ORF) that complement 216. The entire ORF is necessary and capable of encoding a 299-amino-acid protein of 35 kD, sufficient for efficient complementation of the mutation and no transcription termination structures arc evident in strain 216. Some reduction of complementation effi­ immediately downstream of the ORF. No significant ciency was seen with the two farthest upstream dele­ similarities were found using the FASTA program (Pear­ tions that lacked 270 and 730 bases of pWB216S2.4, but son and Lipman 1988) to compare the DNA sequence of the reason for this is unknown. These deletions may hetR to GenBank (release 64), or its predicted protein have fused a vector promoter to the hetR gene, resulting in a lethal phenotype (see below). After determining which deletion subclones could still complement strain 216, we observed that the introduc­ 2.4 kb insert in IIM'W Xha\ fhifW S<v\ pWB216S2.4 tion of other subclones with larger deletions resulted in hclR small numbers of Fix^ colonies on N" plates after sev­ eral weeks. It seemed likely that these colonies were the sc^ result of recombination between the introduced plasmid and the mutant chromosomal hetR gene. We used the frequency of this marker rescue by recombination (as as: reflected in the number of Fix"^ colonies) together with the end points of those deletion subclones that did not Figure 1. Physical map of the 2.4-kb insert in plasmid form these colonies to locate the chromosomal lesion in pWB216S2.4 containing the hetR region. Only selected restric­ tion sites are indicated. Wide horizontal lines indicate the por­ strain 216 (data not shown). These results mapped the tions of the nested deletions that were sequenced. The remain­ mutation to a 160-bp region near the center of the hetR der of each line indicates the DNA remaining in that deletion. gene, as indicated in Figure 1. These deletions were used for complementation tests. If a dele­ A 318-bp region containing the expected mutation in tion complemented, the line is marked with a filled circle. The 216 DNA and the corresponding region of wild-type two deletions that complemented poorly are shown marked DNA were amplified using the polymerase chain reac­ with stippled circles. Deletions that produced only a few tion (PCR). The sequences corresponding to the oligonu­ marker-rescued colonies are shown marked with a half-circle. cleotide primers used are shown in Figure 2. Following Lines marked with open circles indicate deletions that neither complemented nor gave marker-rescued colonies. The marker- symmetric amplification, a portion of the double- rescue results place the mutation in strain 216 within the 160- stranded product was amplified asymmetrically, and the bp region shown between the vertical dotted lines.
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