Fig. 1
The surfactin gene cluster (BGC0000433) in B. velezensis FZB42. Transcription of the srfAA-AD operon is governed by the Psrf promoter. The comS gene is embedded within the srfAB gene, and is also transcribed by the Psrf promoter. Three genes, srfAA, srfAB, and srfAC, transcribe the nonribosomal peptide synthetases (NRPS) SrfAA, SrfAB, SrfAC, and the thioesterase SrfAD. SrfAA contains N-terminally the CS-domain and acylates the first amino acid Glu1 with various fatty acids. The elongation modules of SrfAA, SrfAB, and SrfAC yield the linear heptapeptide indicated at the bottom of the figure. The TE domain in SrfAC releases the lipopeptide and performs the cyclization between Leu7, and the fatty acid chain linked with Glu1. The second TE domain present in SrfAD seems to have mainly repair functions. The sfp gene, located downstream from the srfAA-AD operon encodes a phosphopantetheinyl transferase (PPTase), which is indispensable for nonribosomal synthesis of surfactin, the other lipopeptides (fengycin and bacillomycin D), and polyketides in FZB42. The yczE gene product, a membrane protein with unknown function, was also shown to be essential for synthesis of cyclic lipopeptides