Regulation of the Synthesis and Secretion of the Iron Chelator Cyclodipeptide Pulcherriminic Acid in Bacillus licheniformis .

The cyclodipeptide pulcherriminic acid synthesized by Bacillus licheniformis is an iron chelator and antagonizes certain pathogens by removing iron from the environment. But as the insoluble Fe-pulcherriminic acid complex cannot act as an iron carrier like siderophores, so excessive synthesized pulcherriminic acid causes iron starvation for the producer cells. At present, regulation of pulcherriminic acid synthesis and the mechanism by which B. licheniformis strikes a balance between biocontrol and self-protection from excessive iron removal remain unclear. This study provides insights into the regulation network and explained the decision mechanism of pulcherriminic acid biosynthesis. The synthetic gene cluster yvmC-cypX was directly negatively regulated by three regulators, AbrB, YvnA and YvmB. Inside the regulation network, YvnA expression was not only repressed by AbrB but also repressed by iron-limiting environments, while YvmB expression was repressed by YvnA. The transporter gene yvmA is repressed by YvmB and required for pulcherriminic acid secretion. The biosynthesis window was determined by the combined concentration of the three regulators in iron rich environment. In iron-limiting conditions, cells close the pulcherriminic acid synthesis pathway by down regulating the YvnA expression. IMPORTANCE The cyclodipeptides are widespread in nature, and exhibit a broad variety of biological and pharmacological activities. The cyclodipeptide scaffold is synthesized by non-ribosomal peptide synthetases (NRPSs) and cyclodipeptide synthases (CDPSs). At present, it is clear that CDPSs use aminoacyl tRNAs as substrates to synthesize the two peptide bonds, and the pulcherriminic acid synthase YvmC is a member of the eight identified CDPSs. However, little is known about the regulation of cyclodipeptide synthesis and secretion. In this study, we showed that the AbrB, which is considered to be the main regulator of NRPS-dependent pathways, is also involved in the regulation of CDPS genes. But AbrB is not the decisive factor of pulcherrimin acid synthesis, the expression of YvnA determines the fate of pulcherriminic acid synthesis. With this information on how CDPS gene transcription was regulated, a clearer understanding of cyclodipeptides synthesis can be developed for B. licheniformis Similar approaches could be used to augment our knowledge on CDPS in other bacteria.