Biofilm formation by Bacillus subtilis requires an endoribonuclease-containing multisubunit complex that controls mRNA levels for the matrix gene repressor SinR

Biofilm formation by Bacillus subtilis is largely governed by a circuit in which the response regulator Spo0A turns on the gene for the anti-repressor SinI. SinI, in turn, binds to and inactivates SinR, a dedicated repressor of genes for matrix production. Mutants of the genes ylbF, ymcA and yaaT are blocked in biofilm formation, but the mechanism by which they act has been mysterious. A recent report attributed their role in biofilm formation to stimulating Spo0A activity. However, we detect no measurable effect on the transcription of sinI. Instead, we find that the block in biofilm formation is caused by an increase in the levels of SinR and of its mRNA. Evidence is presented that YlbF, YmcA and YaaT interact with, and control the activity of, RNase Y, which is known to destabilize sinR mRNA. We also show that the processing of another target of RNase Y, cggR-gapA mRNA, similarly depends on YlbF and YmcA. Our work suggests that sinR mRNA stability is an additional posttranscriptional control mechanism governing the switch to multicellularity and raises the possibility that YlbF, YmcA and YaaT broadly regulate mRNA stability as part of an RNase Y-containing, multi-subunit complex.