An autoregulatory circuit affecting peptide signaling in Bacillus subtilis

Abstract

The competence and sporulation factor (CSF) of Bacillus subtilis is an extracellular pentapeptide produced from the product of phrC. CSF has at least three activities: (i) at low concentrations, it stimulates expression of genes activated by the transcription factor ComA; at higher concentrations, it (ii) inhibits expression of those same genes and (iii) stimulates sporulation. Because the activities of CSF are concentration dependent, we measured the amount of extracellular CSF produced by cells. We found that by mid-exponential phase, CSF accumulated to concentrations (1 to 5 nM) that stimulate ComA-dependent gene expression. Upon entry into stationary phase, CSF reached 50 to 100 nM, concentrations that stimulate sporulation and inhibit ComA-dependent gene expression. Transcription of phrC was found to be controlled by two promoters: P1, which precedes rapC, the gene upstream of phrC; and P2, which directs transcription of phrC only. Both RapC and CSF were found to be part of autoregulatory loops that affect transcription from P1, which we show is activated by ComA~P. RapC negatively regulates its own expression, presumably due to its ability to inhibit accumulation of ComA~P. CSF positively regulates its own expression, presumably due to its ability to inhibit RapC activity. Transcription from P2, which is controlled by the alternate sigma factor σ(H), increased as cells entered stationary phase, contributing to the increase in extracellular CSF at this time. In addition to controlling transcription of phrC, σ(H) appears to control expression of at least one other gene required for production of CSF.