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. SigI1 promotes transcription from sigI1 and celS promoters (PubMed:20937888). Negatively regulated by the anti-sigma-I factor RsgI1 (PubMed:20937888). Binding of the polysaccharide substrate to RsgI1 may lead to the release and activation of SigI1 (Probable). Interacts with RsgI1. Up-regulated in the presence of cellulose. Belongs to the sigma-70 factor family. SigI subfamily.