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Nisin induction without nisin secretion

Department of Applied Chemistry and Microbiology, PO Box 56, FI-00014 University of Helsinki, Finland

Correspondence
Per E. J. Saris
per.saris{at}helsinki.fi

Nisin Z, a post-translationally modified antimicrobial peptide of Lactococcus lactis, is positively autoregulated by extracellular nisin via the two-component regulatory proteins NisRK. A mutation in the nisin NisT transporter rendered L. lactis incapable of nisin secretion, and nisin accumulated inside the cells. Normally nisin is activated after secretion by the serine protease NisP in the cell wall. This study showed that when secretion of nisin was blocked, intracellular proteolytic activity could cleave the N-terminal leader peptide of nisin precursor, resulting in active nisin. The isolated cytoplasm of a non-nisin producer could also cleave the leader from the nisin precursor, showing that the cytoplasm of L. lactis cells does contain proteolytic activity capable of cleaving the leader from fully modified nisin precursor. Nisin could not be detected in the growth supernatant of the NisT mutant strain with a nisin-sensing strain (sensitivity 10 pg ml^–1), which has a green fluorescent protein gene connected to the nisin-inducible nisA promoter and a functional nisin signal transduction circuit. Northern analysis of the NisT mutant cells revealed that even though the cells could not secrete nisin, the nisin-inducible promoter P_nisZ was active. In a nisB or nisC background, where nisin could not be fully modified due to the mutations in the nisin modification machinery, the unmodified or partly modified nisin precursor accumulated in the cytoplasm. This immature nisin could not induce the P_nisZ promoter. The results suggest that when active nisin is accumulated in the cytoplasm, it can insert into the membrane and from there extrude parts of the molecule into the pseudoperiplasmic space to interact with the signal-recognition domain of the histidine kinase NisK. Potentially, signal presentation via the membrane represents a general pathway for amphiphilic signals to interact with their sensors for signal transduction.

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