Anti-sigma factor for SigI1. Negatively regulates SigI1 activity through direct interaction (PubMed:20937888). Binding of the polysaccharide substrate to the extracellular C-terminal sensing domain of RsgI1 may induce a conformational change in its N-terminal cytoplasmic region, leading to the release and activation of SigI1 (Probable). Interacts (via RsgI N-terminal anti-sigma domain) with SigI1. The CBM3 domain binds to cellulose.