A major new component in the cellulosome of Clostridium thermocellum is a processive endo-β-1,4-glucanase producing cellotetraose

Cel9R, a major component in the cellulosome of Clostridium thermocellum, is one of the most prevalent β-glucanases in the complex after Cel48S and Cel8A. The recombinant product of gene celR is optimally active at 78.5 °C on amorphous cellulose, carboxymethyl-cellulose, and barley β-1,3-1,4-glucan. From amorphous cellulose it produces initially cellotetraose which is slowly degraded to glucose, cellobiose and cellotriose. This product pattern indicates a processive endoglucanase-mode which was corroborated by the initial and simultaneous production of new reducing ends in the soluble as well as in the insoluble fraction of amorphous cellulose. pNP-Cellopentaoside is degraded to cellotetraose and pNP-glucoside, suggesting cellotetraose release from the non-reducing end. The newly discovered Cel9R thus is a novel type of cellulase in the cellulosome of C. thermocellum: a processive endo-β-1,4-glucanase producing cellotetraose as the primary hydrolysis product. The presence in the cellulosome and the hydrolytic mode of this cellotetraohydrolase has implications for our understanding of the in vivo conversion of cellulose by bacteria.