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 sigI6 itself, xyn11B, xyn10D, xyn10Z, xyn10Y, cel9V, cseP, sigI1, cipA, and rsgI5 (PubMed:26731480). Negatively regulated by the anti-sigma-I factor RsgI6 (By similarity). Binding of the polysaccharide substrate to RsgI6 may lead to the release and activation of SigI6 (By similarity). Interacts with RsgI6. Autoregulated (Probable). Up-regulated in the presence of xylan (PubMed:20937888). Belongs to the sigma-70 factor family. SigI subfamily.