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Xylan Utilization in Human Gut Commensal Bacteria Is Orchestrated by Unique Modular Organization of Polysaccharide-Degrading Enzymes

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Xylan Utilization in Human Gut Commensal Bacteria Is Orchestrated by Unique Modular Organization of Polysaccharide-Degrading Enzymes Xylan utilization in human gut commensal bacteria is orchestrated by unique modular organization of polysaccharide-degrading enzymes Meiling Zhanga,b,c,1,2, Jonathan R. Chekand,1, Dylan Dodda,b,e,1,3, Pei-Ying Hongc,4, Lauren Radlinskia,b,e, Vanessa Revindrana,b, Satish K. Nairb,d,5, Roderick I. Mackiea,b,c,5, and Isaac Canna,b,c,e,5 aEnergy Biosciences Institute, bInstitute for Genomic Biology, and Departments of cAnimal Sciences, dBiochemistry, and eMicrobiology, University of Illinois at Urbana-Champaign, Urbana, IL 61801 Edited by Arnold L. Demain, Drew University, Madison, NJ, and approved July 29, 2014 (received for review April 19, 2014) Enzymes that degrade dietary and host-derived glycans represent One of the most important functional roles specific to the lower the most abundant functional activities encoded by genes unique gut microbiota is the degradation and fermentation of dietary fiber to the human gut microbiome. However, the biochemical activities that is resistant to digestion by human enzymes. The enzymes in- of a vast majority of the glycan-degrading enzymes are poorly volved in breaking down polysaccharides are members of carbo- understood. Here, we use transcriptome sequencing to under- hydrate active enzymes (CAZymes) and are categorized into stand the diversity of genes expressed by the human gut bacteria families of carbohydrate-binding modules (CBMs), carbohydrate Bacteroides intestinalis and Bacteroides ovatus grown in monocul- esterases (CEs), glycoside hydrolases (GHs), glycoside transferases ture with the abundant dietary polysaccharide xylan. The most (GTs), or polysaccharide lyases (PLs) (14). The CAZy gene cat- highly induced carbohydrate active genes encode a unique glycoside alog of the human genome pales in comparison with that of the gut hydrolase (GH) family 10 endoxylanase (BiXyn10A or BACINT_04215 microbiota, which exceeds the total number of human CAZymes and BACOVA_04390) that is highly conserved in the Bacteroidetes by at least 600-fold (15). This observation suggests that humans xylan utilization system. The BiXyn10A modular architecture con- rely heavily on their gut microbes to extract energy from dietary sists of a GH10 catalytic module disrupted by a 250 amino acid se- carbohydrates (16), especially in their lower GIT. Despite the quence of unknown function. Biochemical analysis of BiXyn10A importance of these genes in energy capture by the gut microbiota, demonstrated that such insertion sequences encode a new fam- very little is known of the function of their encoded polypeptides. ily of carbohydrate-binding modules (CBMs) that binds to xy- Xylan is the second most abundant structural polysaccharide in lose-configured oligosaccharide/polysaccharide ligands, the substrate plant cell walls and is particularly high in cereal grains, a significant of the BiXyn10A enzymatic activity. The crystal structures of CBM1 component of the human diet (17). Moreover, the digestibility rates from BiXyn10A (1.8 Å), a cocomplex of BiXyn10A CBM1 with xylohex- of xylan in the human GIT are estimated at about 72% (18). Of the aose (1.14 Å), and the CBM from its homolog in the Prevotella bryantii B14 Xyn10C (1.68 Å) reveal an unanticipated mode for ligand binding. Significance A minimal enzyme mix, composed of the gene products of four of the most highly up-regulated genes during growth on wheat arabinox- Fermentation of dietary fiber in the lower gut of humans is a crit- ylan, depolymerizes the polysaccharide into its component sugars. ical process for the function and integrity of both the bacterial The combined biochemical and biophysical studies presented here community and host cells. Here we demonstrate that two human provide a framework for understanding fiber metabolism by an im- gut commensal Bacteroides are equipped with unique enzymes portant group within the commensal bacterial population known to that allow degradation of xylan, a common hemicellulose in hu- influence human health. man diets. Furthermore, we identify a novel carbohydrate-binding module (CBM) family that disrupts the catalytic domain of a gly- RNAseq | gut microbiota | xylanolytic bacteria | hemicellulose | coside hydrolase 10 (GH10) endoxylanase and facilitates the hy- human nutrition drolytic activity of the enzyme. The conservation of the unique modular architecture of the GH10 endoxylanase in the genomes he human lower gastrointestinal tract (GIT) is home to tril- of diverse Bacteroidetes suggests a critical role in fiber digestion Tlions of bacteria that equip humans with biological activities in this phylum. that are absent in human cells (1–3). The microbes within the human lower gut perform a wide variety of functions important Author contributions: D.D., P.-Y.H., S.K.N., R.I.M., and I.C. designed research; M.Z., J.R.C., for normal physiology, including promoting maturation of the D.D., P.-Y.H., L.R., and V.R. performed research; S.K.N., R.I.M., and I.C. contributed new re- agents/analytic tools; M.Z., J.R.C., D.D., V.R., S.K.N., and I.C. analyzed data; and M.Z., J.R.C., immune system (4), development of the GIT (5), and improving D.D., S.K.N., R.I.M., and I.C. wrote the paper. energy capture from dietary components (6). Moreover, dysbiosis The authors declare no conflict of interest. of the microbiota is associated with various diseases including This article is a PNAS Direct Submission. inflammatory bowel disease (7), obesity (8), and atherosclerotic Data deposition: The atomic coordinates have been deposited in the Protein Data Bank, cardiovascular disease (9, 10). The importance of the microbial www.pdb.org (PDB ID codes 4MGS, 4QPW, and 4MGQ). community in the GIT to human health has prompted analysis of 1M.Z., J.R.C., and D.D. contributed equally to this work. gut bacteria on a massive scale. Using recent advances in DNA 2Present address: School of Life Sciences, East China Normal University, Shanghai 200241, China. sequencing technologies, investigators have captured the diversity 3Present address: Department of Pathology, Stanford University School of Medicine, of organisms in these complex microbial assemblages (11, 12) and Stanford, CA 94304. have generated enormous catalogs of microbial genes (1, 3, 13). 4Present address: Environmental Science and Engineering, Water Desalination and Reuse However, the rapid accumulation of DNA sequences has far Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955- 6900, Saudi Arabia. surpassed our ability to assign function to genes. Indeed, ∼75% of 5To whom correspondence may be addressed. Email: [email protected], [email protected], genes encoded by the gut microbiota have gene products that or [email protected]. cluster into unknown orthologous groups or represent entirely This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. novel protein families (3). 1073/pnas.1406156111/-/DCSupplemental. E3708–E3717 | PNAS | Published online August 18, 2014 www.pnas.org/cgi/doi/10.1073/pnas.1406156111 Downloaded by guest on September 24, 2021 bacteria in the human gut, the genus Bacteroides possesses the most number of B. ovatus xylanolytic genes are induced during growth PNAS PLUS expanded glycolytic gene repertoires that target xylan degradation with xylan (19). Furthermore, a hybrid two-component system (19, 20). These organisms use a highly specialized system consisting (HTCS) regulator whose periplasmic-sensing domain binds xyloo- of a core set of polysaccharide-binding proteins, outer membrane ligosaccharides was identified adjacent to the highly induced xyla- transporters, and glycolytic enzymes to cleave large polysaccharides nolytic genes, suggesting a role in the transcriptional response to into oligosaccharides that are then transported by TonB-dependent xylan (19). Despite the vast diversity of xylan-degrading genes in transporters into the periplasm. An arsenal of glycolytic enzymes these two organisms, very little is known about the specific roles of then converts the oligosaccharides into fermentable monosaccha- the individual genes in xylan utilization. rides (21). The xylan utilization system (XUS) appears to be con- In the current study, we used whole-genome transcriptome served across xylan-degrading Bacteroidetes from different gas- sequencing to guide the study of genes with roles in xylan deg- trointestinal systems (20, 22), but the roles of individual genes in radation by the two human gut commensal bacteria B. intestinalis the gene cluster have not been defined. DSM 17393 and B. ovatus ATCC 8483. The most highly up- Xylan degradation is not universally shared by the Bacte- regulated genes encode BiXyn10A and BACOVA_04390, two roidetes in the human gut; rather, this capacity varies among polypeptides with an unusual modular organization that is different species and even between different strains. As recently widespread, especially in the human lower gut Bacteroidetes. reviewed (20), human gut Bacteroides that have the capacity to The transcriptome data from another gut Bacteroidetes (the degrade xylan include strains of Bacteroides eggerthii, Bacteroides ruminal bacterium Prevotella bryantii B14) showed that a gene cellulosilyticus, Bacteroides intestinalis, Bacteroides ovatus, and encoding a polypeptide with similar architecture to BiXyn10A Bacteroides xylanisolvens. Partial genome sequences for several and BACOVA_04390 is the most highly up-regulated CAZy strains of these bacteria are present in the publicly available gene when the bacterium is metabolizing xylan (22). BiXyn10A
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