Positive, redox-sensitive regulator of sigma-F (SigF) activity. When reduced binds to anti-sigma-F factor RsbW (UsfX) preventing its binding to SigF, thus activating transcription. Monomer. Interacts with anti-sigma-F factor RsbW (UsfX) under reducing conditions, with a possible 2:1 RbsW:RsfA stoichiometry. Belongs to the anti-sigma-factor antagonist family.