The heat shock genes in B. subtilis can be classified into several groups according to their regulation [1], and the sigma gene, sigI, of Bacillus subtilis belongs to the group IV heat-shock response genes and has many orthologues in the bacterial phylum Firmicutes [1]. Regulation of sigma factor I is carried out by RsgI from the same operon, and this N-terminal cytoplasmic portion of RsgI ('upstream' of the single transmembrane helix) has been shown to interact directly with Sigma-I [2].

1: Putative sigma factor SigI (YkoZ) of Bacillus subtilis is induced by heat shock. Zuber U, Drzewiecki K, Hecker M; J Bacteriol. 2001;183:1472-1475. PMID:11157964

2: Regulatory role of RsgI in sigI expression in Bacillus subtilis. Asai K, Ootsuji T, Obata K, Matsumoto T, Fujita Y, Sadaie Y; Microbiology. 2007;153:92-101. PMID:17185538