Positive regulator of sigma-F (SigF) activity. Binds to anti-sigma-F factor RsbW (UsfX) preventing its binding to SigF, thus activating transcription. Interacts with anti-sigma-F factor RsbW (UsfX). Its phosphorylation may prevent this interaction. Putative phosphorylation on Ser-61 may prevent interaction with RsbW. Belongs to the anti-sigma-factor antagonist family.