A negative feedback loop that limits the ectopic activation of a cell type-specific sporulation sigma factor of bacillus subtilis

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. © 2011 serrano et al.