Reversal of an epigenetic switch governing cell chaining in Bacillus subtilis by protein instability

Summary Bacillus subtilis forms long chains of cells during growth and biofilm formation. Cell separation is mediated by autolysins, whose genes are under the negative control of a heteromeric complex composed of the proteins SinR and SlrR. Formation of the SinR–SlrR complex is governed by a self‐reinforcing, double‐negative feedback loop in which SinR represses the gene for SlrR and SlrR, by forming the SinR–SlrR complex, titrates SinR and prevents it from repressing slrR . The loop is a bistable switch and exists in a SlrR LOW state in which autolysin genes are on, and a SlrR HIGH state in which autolysin genes are repressed by SinR–SlrR. Cells in the SlrR LOW state are driven into the SlrR HIGH state by SinI, an antirepressor that binds to and inhibits SinR. However, the mechanism by which cells in the SlrR HIGH state revert back to the SlrR LOW state is unknown. We report that SlrR is proteolytically unstable and present evidence that self‐cleavage via a LexA‐like autopeptidase and ClpC contribute to its degradation. Cells producing a self‐cleavage‐resistant mutant of SlrR exhibited more persistent chaining during growth and yielded biofilms with enhanced structural complexity. We propose that degradation of SlrR allows cells to switch from the SlrR HIGH to the SlrR LOW state.