Fig. 1

The schematic model for the regulation of the transcription of the srf operon network involved in two extracellular signaling peptide-mediated quorum sensing in B. subtilis. The srf operon was composed of four genes encoding surfactin synthetase with comS embedded between gene srfAA and srfAB. Both ComX and CSF activate the transcription of the srf operon by stimulating the activity of transcription factor ComA through phosphorylation (ComA-P) via two separate pathways, one of which requires the ComX peptide to be processed outside by ComQ. Another pathway is mediated by CSF, which is encoded by phrC and is initially imported as inactive-form CSF (inact). The active-form CSF (act), which interacts with two intracellular targets, is generated through processing by the Spo0K gene. At low concentrations, CSF inhibits the activity of an aspartyl-phosphate phosphatase, RapC, and is speculated to inhibit the kinase activity of ComP at high concentrations