Transcription analysis of the nisin gene cluster in Lactococcus lactis ATCC 11454

The biosynthesis of the antimicrobial peptide nisin by Lactococcus lactis ATCC 11454 is subject to autoregulation. Transcriptions of nisABTCIP, where nisI encodes an immunity protein, and nisFEG, which encode an ABC transporter, are tightly induced by external nisin via the two-component system NisRK. One objective of this thesis is to study how sufficient immunity (NisI) can be expressed when the cell first encounters external nisin. In this study, Northern-blot and RT-PCR showed that nisI mRNA was present independent of nisA mRNA. Further mRNA stability study demonstrated that nisI mRNA had a much shorter half-life compared to nisA mRNA, suggesting that an internal promoter within nisABTCIP operon caused the independent nisI transcription that further provided cell immunity prior to the initiation of nisin auto-induction. The promoter was confirmed when the adjacent upstream region of nisI was fused to a probe vector, exhibiting constitutive promoter activity. The transcription start site (TSS) of the promoter was further mapped at 123 bp upstream of the ATG codon. Ordered 5' deletions revealed that transcription activation depended on sequences located up to -234 bp from the TSS.; Another objective of this thesis is to study the repressive mechanism of high growth temperature on nisin production. RT-PCR and Western-hybridization showed that nisR was transcribed and translated while nisA was repressed, eliminating the possibility that lack of NisRK caused the loss of nisA transcription. Gel shift experiments demonstrated no nisA-binding proteins present in cell extracts following a growth at 40°C, indicating lack of activated NisR at high growth temperature. Heparin column purification of nisA binding proteins and isolation of total phosphorylated proteins substantiated this finding as neither other repressor(s) nor phosphorylated NisR could be detected at 40°C. Therefore, it could be concluded that the absence of nisA transcription at 40°C was due to a lack of phosphorylated NisR which is needed to activate transcription of nisA. Further analysis of the NisK kinase activity at 40°C showed a significant repression, elucidating the reason of lacking phosphorylated NisR at 40°C. The relationship between reduced NisK kinase and cell morphology change was also discussed.