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 RsgI4 (By similarity). Binding of the polysaccharide substrate to RsgI4 may lead to the release and activation of SigI4 (By similarity). Interacts with RsgI4. Up-regulated in the presence of cellulose and further up-regulated in the presence of both cellulose and xylan (PubMed:20937888). Up-regulated in pretreated yellow poplar (PYP)-grown cells (PubMed:24782837). Belongs to the sigma-70 factor family. SigI subfamily.