Anti-sigma factor for SigI3. Negatively regulates SigI3 activity through direct interaction. Binding of the polysaccharide substrate to the extracellular C-terminal sensing domain of RsgI3 may induce a conformational change in its N-terminal cytoplasmic region, leading to the release and activation of SigI3. Interacts (via RsgI N-terminal anti-sigma domain) with SigI3. Up-regulated in pretreated yellow poplar (PYP)-grown cells. The PA14 dyad binds strongly to pectin as well as to polygalacturonic acid and alfalfa cell walls, but to a lesser extent.