Three cellulosomal xylanase genes in Clostridium thermocellum are regulated by both vegetative SigA (σA) and alternative SigI6 (σI6) factors

Clostridium thermocellum efficiently degrades crystalline cellulose by a high molecular weight protein complex, the cellulosome. The bacterium regulates its cellulosomal genes using a unique extracellular biomass-sensing mechanism that involves alternative sigma factors and extracellular carbohydrate-binding modules attached to intracellular anti-sigma domains. In this study, we identified three cellulosomal xylanase genes that are regulated by the σI6/RsgI6 system by utilizing sigI6 and rsgI6 knockout mutants together with primer extension analysis. Our results indicate that cellulosomal genes are expressed from both alternative σI6 and σA vegetative promoters.