Autoregulation of Nisin Biosynthesis in Lactococcus lactis by Signal Transduction

The post-translationally modified, antimicrobial peptide nisin is secreted by strains of Lactococcus lactis that contain the chromosomally located nisin biosynthetic gene cluster nisABTCIPRKFEG. When a 4-base pair deletion is introduced into the structural nisA gene (delta nisA), transcription of delta nisA is abolished. Transcription of the delta nisA gene is restored by adding subinhibitory amounts of nisin, nisin mutants, or nisin analogs to the culture medium, but not by the unmodified precursor peptide or by several other antimicrobial peptides. Upon disruption of the nisK gene, which encodes a putative sensor protein that belongs to the class of two-component regulators, transcription of delta nisA was no longer inducible by nisin. Fusion of a nisA promoter fragment to the promoterless reporter gene gusA resulted in expression of gusA in L. lactis NZ9800 (delta nisA) only upon induction with nisin species. The expression level of gusA was directly related to the amount of inducer that was added extracellularly. These results provide insight into a new mechanism of autoregulation through signal transduction in prokaryotes and demonstrate that antimicrobial peptides can exert a second function as signaling molecules.