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). Recognizes the predicted promoters associated with sigI3 itself, pl11, ce12 and cipA (PubMed:26731480). Negatively regulated by the anti-sigma-I factor RsgI3 (By similarity). Binding of the polysaccharide substrate to RsgI3 may lead to the release and activation of SigI3 (By similarity). Interacts with RsgI3. Autoregulated (Probable). Up-regulated in the presence of xylan (PubMed:20937888). Up-regulated in pretreated yellow poplar (PYP)-grown cells (PubMed:24782837). Belongs to the sigma-70 factor family. SigI subfamily.