Bacteriocin lactocin-S

Lactocin S, a bacteriocin produced by Lactobacillus sake L45, has been purified to homogeneity by ion exchange, hydrophobic interaction and reverse-phase chromatography, and gel filtration. The purification resulted in approximately a 40,000-fold increase in the specific activity of lactocin S and enabled the determination of a major part of the amino acid sequence. Judging from the amino acid composition, lactocin S contained approximately 33 amino acid residues, of which about 50% were the nonpolar amino acids alanine, valine, and leucine. Amino acids were not detected upon direct N-terminal sequencing, indicating that the N-terminal amino acid was blocked. By cyanogen bromide cleavage at an internal methionine, the sequence of the 25 amino acids (including the methionine at the cleavage site) in the C-terminal part of the molecule was determined. The sequence was Met-Glu-Leu-Leu-Pro-Thr-Ala-Ala-Val-Leu-Tyr-Xaa-Asp-Val-Ala-Gly-Xaa-Phe- Lys-Tyr-Xaa-Ala-Lys-His-His, where Xaa represents unidentified residues. It is likely that the unidentified residues are modified forms of cysteine or amino acids associated with cysteine, since two cysteic acids per lactocin S molecule were found upon performic acid oxidation of lactocin S. The sequence was unique when compared to the SWISS-PROT data bank.

The lactocin S structural gene (lasA) in seven bacteriocinogenic lactobacilli isolated from fermented sausages was studied. Two degenerate primers were synthesized to amplify a 75-bp fragment of the gene. Three strains amplified the fragment from their plasmid DNA, and hybridization analysis confirmed these results.

Strains of Enterococcus faecalis and Lactobacillus sake have been found to express lantibiotics with unusual properties. The enterococcal lantibiotic is unusual in that it consists of two dissimilar subunits, both putatively containing modifications consistent with those found in other lantibiotics. The enterococcal lantibiotic is also unusual in the number of proteolytic steps involved in secretion signal removal and activation. Moreover, it has been observed to contribute to enterococcal disease in humans and in animal models. Structural studies of lactocin S, expressed by a strain of L. sake highlight unique properties including the presence of D-alanine within its structure, and a protease putatively responsible for lactocin S secretion signal peptide removal which, itself, lacks a signal or propeptide sequence. Despite the unusual properties of each of these lantibiotics, the operons encoding each, and accompanying auxiliary functions, are collinear suggesting a common ancestry. The accretion of interdigitating DNA sequences between genes encoded within the lactocin S determinant are unique to that determinant, however, and are of unknown function.