Transcriptional downregulator hexS controlling prodigiosin and serrawettin W1 biosynthesis in Serratia marcescens

Serratia marcescens has been known as a temperature-dependent producer of two chemically different exolipids (red pigment prodigiosin and biosurfactant serrawettin W1) in parallel. During genetic investigation of such control mechanisms, mini-Tn 5 insertional mutant Tan1 overproducing these exolipids was isolated. The gene concerning such disregulation was identified as hexS by DNA cloning followed by sequencing and homology analysis of the presumed product with 314 amino-acids. The product HexS was the homologue of HexA of Erwinia carotovora ssp. carotovora and classified as a transcriptional regulator belonging to LysR family. By RT-PCR analysis, the hexS mutant was shown to over-transcribe the pigA gene (the first gene of the pig cluster involved in prodigiosin synthesis) and the swrW gene encoding serrawettin W1 synthetase belonging to the nonribosomal peptide synthetase family. In contrast, transcription of the pswP gene encoding phosphopantetheinyl transferase in Tan1 was in the level of parent strain 274. Purified protein encoded in his(6)-hexS demonstrated binding activity to DNA fragments of the upstream region of pigA and swrW genes and not to that of the pswP gene. S. marcescens strain 274 transformed with a low-copy plasmid carrying hexS demonstrated reduced production of prodigiosin and serrawettin W1, and reduced activity of exoenzymes (protease, chitinase, and DNase) except phospholipase C. Possible generation of virulent S. marcescens by derepression or mutation of the hexS gene in infected tissues or ex vivo environments was suggested.