SlrA/SlrR/SinR inhibits motility gene expression upstream of a hypersensitive and hysteric switch at the level of σD 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, 25 kb 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 could be bypassed by artificial ectopic expression of sigD that 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 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.