Background (Pseudo) Bacteroides cellulosolvens is an anaerobic, mesophilic, cellulolytic, cellulosome-producing clostridial bacterium capable of utilizing cellulose and cellobiose as carbon sources. Recently, we sequenced the B. cellulosolvens genome, and subsequent comprehensive bioinformatic analysis, herein reported, revealed an unprecedented number of cellulosome-related components, including 78 cohesin modules scattered among 31 scaffoldins and more than 200 dockerin-bearing ORFs. In terms of numbers, the B. cellulosolvens cellulosome system represents the most intricate, compositionally diverse cellulosome system yet known in nature.ResultsThe organization of the B. cellulosolvens cellulosome is unique compared to previously described cellulosome systems. In contrast to all other known cellulosomes, the cohesin types are reversed for all scaffoldins i.e., the type II cohesins are located on the enzyme-integrating primary scaffoldin, whereas the type I cohesins are located on the anchoring scaffoldins. Many of the type II dockerin-bearing ORFs include X60 modules, which are known to stabilize type II cohesin–dockerin interactions. In the present work, we focused on revealing the architectural arrangement of cellulosome structure in this bacterium by examining numerous interactions between the various cohesin and dockerin modules. In total, we cloned and expressed 43 representative cohesins and 27 dockerins. The results revealed various possible architectures of cell-anchored and cell-free cellulosomes, which serve to assemble distinctive cellulosome types via three distinct cohesin–dockerin specificities: type I, type II, and a novel-type designated R (distinct from type III interactions, predominant in ruminococcal cellulosomes).ConclusionsThe results of this study provide novel insight into the architecture and function of the most intricate and extensive cellulosomal system known today, thereby extending significantly our overall knowledge base of cellulosome systems and their components. The robust cellulosome system of B. cellulosolvens, with its unique binding specificities and reversal of cohesin–dockerin types, has served to amend our view of the cellulosome paradigm. Revealing new cellulosomal interactions and arrangements is critical for designing high-efficiency artificial cellulosomes for conversion of plant-derived cellulosic biomass towards improved production of biofuels.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-017-0898-6) contains supplementary material, which is available to authorized users.

We report the single-contig genome sequence of the anaerobic, mesophilic, cellulolytic bacterium, Bacteroides cellulosolvens. The bacterium produces a particularly elaborate cellulosome system, wherein the types of cohesin-dockerin interactions are opposite of other known cellulosome systems: cell-surface attachment is thus mediated via type-I interactions, whereas enzymes are integrated via type-II interactions.

Background: Cellulosomal cohesin-dockerin types are reversed in Bacteroides cellulosolvens. Results: Combined crystallographic and computational approaches of a lone cohesin yielded a structural model of the cohesindockerin complex that was verified experimentally. Conclusion: The dockerin dual-binding mode is not exclusive to enzyme integration into cellulosomes; it also characterizes cell-surface attachment. Significance: This combined approach provides a platform for generating testable hypotheses of the high affinity cohesindockerin interaction.