Characterization of all family-9 glycoside hydrolases synthesized by the cellulosome-producing bacterium Clostridium cellulolyticum.

The genome of Clostridium cellulolyticum encodes 13 GH9 enzymes that display seven distinct domain organizations. All but one contain a dockerin module and were formerly detected in the cellulosomes, but only three of them were previously studied (Cel9E, Cel9G, and Cel9M). In this study, the 10 uncharacterized GH9 enzymes were overproduced in Escherichia coli and purified, and their activity pattern was investigated in the free state or in cellulosome chimeras with key cellulosomal cellulases. The newly purified GH9 enzymes, including those that share similar organization, all exhibited distinct activity patterns, various binding capacities on cellulosic substrates, and different synergies with pivotal cellulases in mini-cellulosomes. Furthermore, one enzyme (Cel9X) was characterized as the first genuine endoxyloglucanase belonging to this family, with no activity on soluble and insoluble celluloses. Another GH9 enzyme (Cel9V), whose sequence is 78% identical to the cellulosomal cellulase Cel9E, was found inactive in the free and complexed states on all tested substrates. The sole noncellulosomal GH9 (Cel9W) is a cellulase displaying a broad substrate specificity, whose engineered form bearing a dockerin can act synergistically in minicomplexes. Finally, incorporation of all GH9 cellulases in trivalent cellulosome chimera containing Cel48F and Cel9G generated a mixture of heterogeneous mini-cellulosomes that exhibit more activity on crystalline cellulose than the best homogeneous tri-functional complex. Altogether, our data emphasize the importance of GH9 diversity in bacterial cellulosomes, confirm that Cel9G is the most synergistic GH9 with the major endoprocessive cellulase Cel48F, but also identify Cel9U as an important cellulosomal component during cellulose depolymerization.

Restoration of cellulase activity in the inactive cellulosomal protein Cel9V from Ruminiclostridium cellulolyticum.

Ruminiclostridium cellulolyticum produces extracellular cellulosomes which contain interalia numerous family-9 glycoside hydrolases, including the inactive Cel9V. The latter shares the same organization and 79% sequence identity with the active cellulase Cel9E. Nevertheless, two aromatic residues and a four-residue stretch putatively critical for the activity are missing in Cel9V. Introduction of one Trytophan and the four-residue stretch restored some weak activity in Cel9V, whereas the replacement of its catalytic domain by that of Cel9E generated a fully active cellulase. Altogether our data indicate that a series of mutations in the catalytic domain of Cel9V lead to an essentially inactive cellulase.

Mechanisms involved in xyloglucan catabolism by the cellulosome-producing bacterium Ruminiclostridium cellulolyticum.

Xyloglucan, a ubiquitous highly branched plant polysaccharide, was found to be rapidly degraded and metabolized by the cellulosome-producing bacterium Ruminiclostridium cellulolyticum. Our study shows that at least four cellulosomal enzymes displaying either endo- or exoxyloglucanase activities, achieve the extracellular degradation of xyloglucan into 4-glucosyl backbone xyloglucan oligosaccharides. The released oligosaccharides (composed of up to 9 monosaccharides) are subsequently imported by a highly specific ATP-binding cassette transporter (ABC-transporter), the expression of the corresponding genes being strongly induced by xyloglucan. This polysaccharide also triggers the synthesis of cytoplasmic ß-galactosidase, α-xylosidase, and ß-glucosidase that act sequentially to convert the imported oligosaccharides into galactose, xylose, glucose and unexpectedly cellobiose. Thus R. cellulolyticum has developed an energy-saving strategy to metabolize this hemicellulosic polysaccharide that relies on the action of the extracellular cellulosomes, a highly specialized ABC-transporter, and cytoplasmic enzymes acting in a specific order. This strategy appears to be widespread among cellulosome-producing mesophilic bacteria which display highly similar gene clusters encoding the cytosolic enzymes and the ABC-transporter.

Cel5I, a SLH-Containing Glycoside Hydrolase: Characterization and Investigation on Its Role in Ruminiclostridium cellulolyticum.

Ruminiclostridium cellulolyticum (Clostridium cellulolyticum) is a mesophilic cellulolytic anaerobic bacterium that produces a multi-enzymatic system composed of cellulosomes and non-cellulosomal enzymes to degrade plant cell wall polysaccharides. We characterized one of the non-cellulosomal enzymes, Cel5I, composed of a Family-5 Glycoside Hydrolase catalytic module (GH5), a tandem of Family-17 and -28 Carbohydrate Binding Modules (CBM), and three S-layer homologous (SLH) modules, where the latter are expected to anchor the protein on the cell surface. Cel5I is the only putative endoglucanase targeting the cell surface as well as the only putative protein in R. cellulolyticum containing CBM17 and/or CBM28 modules. We characterized different recombinant structural variants from Cel5I. We showed that Cel5I has an affinity for insoluble cellulosic substrates through its CBMs, that it is the most active endoglucanase on crystalline cellulose of R. cellulolyticum characterized to date and mostly localized in the cell envelope of R. cellulolyticum. Its role in vivo was analyzed using a R. cellulolyticum cel5I mutant strain. Absence of Cel5I in the cell envelope did not lead to a significant variation of the phenotype compared to the wild type strain. Neither in terms of cell binding to cellulose, nor for its growth on crystalline cellulose, thus indicating that the protein has a rather subtle role in tested conditions. Cel5I might be more important in a natural environment, at low concentration of degradable glucose polymers, where its role might be to generate higher concentration of short cellodextrins close to the cell surface, facilitating their uptake or for signalization purpose.

Combining free and aggregated cellulolytic systems in the cellulosome-producing bacterium Ruminiclostridium cellulolyticum.

BACKGROUND: Ruminiclostridium cellulolyticum and Lachnoclostridium phytofermentans (formerly known as Clostridium cellulolyticum and Clostridium phytofermentans, respectively) are anaerobic bacteria that developed different strategies to depolymerize the cellulose and the related plant cell wall polysaccharides. Thus, R. cellulolyticum produces large extracellular multi-enzyme complexes termed cellulosomes, while L. phytofermentans secretes in the environment some cellulose-degrading enzymes as free enzymes. In the present study, the major cellulase from L. phytofermentans was introduced as a free enzyme or as a cellulosomal component in R. cellulolyticum to improve its cellulolytic capacities. RESULTS: The gene at locus Cphy_3367 encoding the major cellulase Cel9A from L. phytofermentans and an engineered gene coding for a modified enzyme harboring a R. cellulolyticum C-terminal dockerin were cloned in an expression vector. After electrotransformation of R. cellulolyticum, both forms of Cel9A were found to be secreted by the corresponding recombinant strains. On minimal medium containing microcrystalline cellulose as the sole source of carbon, the strain secreting the free Cel9A started to grow sooner and consumed cellulose faster than the strain producing the cellulosomal form of Cel9A, or the control strain carrying an empty expression vector. All strains reached the same final cell density but the strain producing the cellulosomal form of Cel9A was unable to completely consume the available cellulose even after an extended cultivation time, conversely to the two other strains. Analyses of their cellulosomes showed that the engineered form of Cel9A bearing a dockerin was successfully incorporated in the complexes, but its integration induced an important release of regular cellulosomal components such as the major cellulase Cel48F, which severely impaired the activity of the complexes on cellulose. In contrast, the cellulosomes synthesized by the control and the free Cel9A-secreting strains displayed similar composition and activity. Finally, the most cellulolytic strain secreting free Cel9A, was also characterized by an early production of lactate, acetate and ethanol as compared to the control strain. CONCLUSIONS: Our study shows that the cellulolytic capacity of R. cellulolyticum can be augmented by supplementing the cellulosomes with a free cellulase originating from L. phytofermentans, whereas integration of the heterologous enzyme in the cellulosomes is rather unfavorable.