MULTIPLE Ruminiclostridium thermocellum ALTERNATIVE σ FACTORS AND THEIR ROLES IN THE REGULATION OF THE CELLULOSOME

Ruminiclostridium thermocellum is a thermophile capable of converting cellulosic wastes into fermentation products such as ethanol. This bacterium secretes a multi-enzyme system called the cellulosome which consists of a nonhydrolytic scaffoldin subunit, CipA, that integrates various catalytic subunits as cellulases, hemicellulases and pectin-degrading enzymes. The enzymatic composition of the cellulosome is regulated according to the type of polysaccharide(s) present in the environment.

Recently, our research groups discovered a set of eight alternative σ factors, related to the Bacillus subtilis σ^I, which may play a role in regulating cellulosomal genes in R. thermocellum (Kahel-Raifer et al. 2010 FEMS Microbiol Lett; Nataf et al. 2010 PNAS USA). Each alternative σ^I–like factors have cognate membrane-associated anti-σ factors (RsgI), and six of them contain carboxyl-terminal carbohydrate binding-modules (CBMs) exposed on the cell surface (Kahel-Raifer et al. 2010 FEMS Microbiol Lett; Bahari et al. 2011 J Ind Microbiol Biotechnol). These RsgI-CBM-domains have been proposed to be polysaccharide sensors. The model suggests that when the RsgI-CBM-domain binds to its target polysaccharide, the RsgI-cytoplasmic-domain releases its cognate σ^I, allowing the activation of its respective regulon.

In the last years, our research groups have focused on demonstrating that individual R. thermocellum σ^I factors regulate specific cellulosomal genes. We found that σ^I6 and σ^I3 are implicated in the regulation of xylanolytic and pectinolytic enzymes, respectively (Sand et al. 2015 FEBS Lett; Muñoz-Gutiérrez and Ortiz-de-Ora et al. 2015 Plos One Submitted; respectively). Interestingly, both, σ^I6 and σ^I3, were able to activate a predicted cipA σ^I-like promoter. Our data supports our hypothesis that several crucial cellulosomal genes are regulated by alternative σ^I factors and their genetic modification may have important future biotechnological impact.