SlrA/SinR/SlrR inhibits motility gene expression upstream of a hypersensitive and hysteretic switch at the level of σ ^D in Bacillus subtilis

Exponentially growing Bacillus subtilis cultures are epigenetically differentiated into two subpopulations in which cells are either ON or OFF for σ ^d-dependent gene expression: a pattern suggestive of bistability. The gene encoding σ ^D, sigD, is part of the 31-gene fla/che operon where its location at the 3' end, 25kb away from the strong P _fla/che promoter, determines its expression level relative to a threshold. Here we show that addition of a single extra copy of the slrA gene in the chromosome inhibited σ ^d-dependent gene expression. SlrA together with SinR and SlrR reduced sigD transcript by potentiating a distance-dependent decrease in fla/che operon transcript abundance that was not mediated by changes in expression from the P _fla/che promoter. Consistent with acting upstream of σ ^D, SlrA/SinR/SlrR was bypassed by artificial ectopic expression of sigD and hysteretically maintained for 20 generations by engaging the sigD gene at the native locus. SlrA/SinR/SlrR was also bypassed by increasing fla/che transcription and resulted in a hypersensitive output in flagellin expression. Thus, flagellin gene expression demonstrated hypersensitivity and hysteresis and we conclude that σ ^d-dependent gene expression is bistable.