Functional identification of the <italic>prnABCD</italic> operon and its regulation in <italic>Serratia plymuthica</italic>.

The antibiotic pyrrolnitrin (PRN) is a tryptophan-derived secondary metabolite that plays an important role in the biocontrol of plant diseases due to its broad-spectrum of antimicrobial activities. The PRN biosynthetic gene cluster remains to be characterised in <italic>Serratia plymuthica</italic>, though it is highly conserved in PRN-producing bacteria. To better understand PRN biosynthesis and its regulation in <italic>Serratia</italic>, the <italic>prnABCD</italic> operon from <italic>S. plymuthica</italic> G3 was cloned, sequenced and expressed in <italic>Escherichia coli</italic> DH5α. Furthermore, an engineered strain <italic>prn</italic>ind which is a conditional mutant of G3 <italic>prnABCD</italic> under the control of the P<italic>tac</italic> promoter was constructed. This mutant was able to overproduce PRN with isopropylthiogalactoside (IPTG) induction by overexpressing <italic>prnABCD</italic>, whilst behaving as a conditional mutant of G3 <italic>prnABCD</italic> in the absence of IPTG. These results confirmed that <italic>prnABCD</italic> is responsible for PRN biosynthesis in strain G3. Further experiments involving <italic>lux-/dsRed</italic>-based promoter fusions, combined with site-directed mutagenesis of the putative σS extended -10 region in the <italic>prnA</italic> promoter, and liquid chromatography-mass spectrometry (LC-MS) analysis extended our previous knowledge about G3, revealing that quorum sensing (QS) regulates PRN biosynthesis through cross talk with RpoS, which may directly activated <italic>prnABCD</italic> transcription. These findings suggest that PRN in <italic>S. plymuthica</italic> G3 is produced in a tightly controlled manner, and has diverse functions, such as modulation of cell motility, in addition to antimicrobial activities. Meanwhile, the construction of inducible mutants could be a powerful tool to improve PRN production, beyond its potential use for the investigation of the biological function of PRN. [ABSTRACT FROM AUTHOR]