Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites and are then released (By similarity). This sigma factor is involved in regulation of cellulosomal genes via an external polysaccharide-sensing mechanism (Probable). Negatively regulated by the anti-sigma-I factor RsgI5 (By similarity). Binding of the polysaccharide substrate to RsgI5 may lead to the release and activation of SigI5 (By similarity). Interacts with RsgI5. Up-regulated in the presence of xylan. Belongs to the sigma-70 factor family. SigI subfamily.