New model of Clostridium thermocellum cellulosome systems and their contribution to cellulose degradation

Clostridium thermocellum is the best microorganism which could degrade cellulose efficiently in nature. Its extremely high cellulose digestion capability is based on its cellulase systems consisting of a free-enzyme system and cellulosome systems in which cellulases are organized on non-catalytic primary scaffoldins to generate large protein complexes (supra-molecular protein machines). These cellulosomal complexes are generally considered to be bound to the cell wall of the organism through another category of scaffoldins (secondary scaffoldins) that assemble the enzyme-laden primary scaffoldins into still larger complexes. In this study, we discovered another type of C. thermocellum cellulosome system, based on a secondary scaffoldin that does not link to the bacterial cell wall, that we term the  “cell-free” cellulosome system in order to distinguish it from the previously proposed cell-bound cellulosome systems. Contribution of these two types of cellulosome systems to the efficient degradation of cellulose is  elucidated via the characterization of mutants having deletions of multiple scaffoldin genes.