This family includes members such as Sigma M inhibitor proteins YhdK. In Bacillus subtilis, sigM (yhdM) gene, is required for growth and survival after salt stress. Expression of sigM is positively autoregulated and is controlled by growth phase and medium composition. SigM-dependent transcription is regulated by the products of both the yhdL and the yhdK genes, which are co-transcribed with the sigM gene [1]. The small hydrophobic protein YhdK, appears to interact with the trans-membrane domain of YhdL, suggesting some specific role for YhdK in the anti-sigma function of YhdL [2].

1: Sigma M, an ECF RNA polymerase sigma factor of Bacillus subtilis 168, is essential for growth and survival in high concentrations of salt. Horsburgh MJ, Moir A; Mol Microbiol. 1999;32:41-50. PMID:10216858

2: Interaction of Bacillus subtilis extracytoplasmic function (ECF) sigma factors with the N-terminal regions of their potential anti-sigma factors. Yoshimura M, Asai K, Sadaie Y, Yoshikawa H; Microbiology. 2004;150:591-599. PMID:14993308