This family includes CC3252 (NrsF- Negative regulator of sigma F) from Caulobacter crescentus, which is proposed to be an anti-sigma factor. In particular, overexpression of NrsF prevents the induction of the sigF regulon after dichromate stress [1]. Family members also include the anti-sigma factor OsrA (blr3039) from Bradyrhizobium japonicum, which is involved in sensing reactive oxygen species. There is evidence that OsrA binds an ECF-type sigma factor (EcfF) in vivo. Furthermore, the conserved cysteine residues 129 and 179 of OsrA are shown to be required for normal function [2]. Family members have 6 predicted transmembrane regions.

1: Extracytoplasmic function (ECF) sigma factor sigmaF is involved in Caulobacter crescentus response to heavy metal stress. Kohler C, Lourenco RF, Avelar GM, Gomes SL; BMC Microbiol. 2012;12:210. PMID:22985357

2: Reactive oxygen species-inducible ECF sigma factors of Bradyrhizobium japonicum. Masloboeva N, Reutimann L, Stiefel P, Follador R, Leimer N, Hennecke H, Mesa S, Fischer HM; PLoS One. 2012;7:e43421. PMID:22916258