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Two highly similar RNA polymerase sigma subunits, σ F  and σ G , govern the early and late phases of forespore-specific gene expression during spore differentiation in  Bacillus subtilis . σ F  drives synthesis of σ G  but the latter only becomes active once engulfment of the forespore by the mother cell is completed, its levels rising quickly due to a positive feedback loop. The mechanisms that prevent premature or ectopic activation of σ G  while discriminating between σ F  and σ G  in the forespore are not fully comprehended. Here, we report that the substitution of an asparagine by a glutamic acid at position 45 of σ G  (N45E) strongly reduced binding by a previously characterized anti-sigma factor, CsfB (also known as Gin), in vitro, and increased the activity of σ G  in vivo. The N45E mutation caused the appearance of a sub-population of pre-divisional cells with strong activity of σ G . CsfB is normally produced in the forespore, under σ F  control, but  sigGN45E  mutant cells also expressed  csfB  and did so in a σ G -dependent manner, autonomously from σ F . Thus, a negative feedback loop involving CsfB counteracts the positive feedback loop resulting from ectopic σ G  activity. N45 is invariant in the homologous position of σ G  orthologues, whereas its functional equivalent in σ F  proteins, E39, is highly conserved. While CsfB does not bind to wild-type σ F , a E39N substitution in σ F  resulted in efficient binding of CsfB to σ F . Moreover, under certain conditions, the E39N alteration strongly restrains the activity of σ F  in vivo, in a  csfB -dependent manner, and the efficiency of sporulation. Therefore, a single amino residue, N45/E39, is sufficient for the ability of CsfB to discriminate between the two forespore-specific sigma factors in  B. subtilis .

A Negative Feedback Loop That Limits the Ectopic Activation of a Cell Type–Specific Sporulation Sigma Factor of Bacillus subtilis