Molecular cloning, sequencing, and expression of lytM, a unique autolytic gene of Staphylococcus aureus

A gene encoding an autolytic activity was identified in an autolysis-deficient mutant (Lyt-) of Staphylococcus aureus which produces only a single band in autolytic-activity gels (N. Mani, P. Tobin, and R. K. Jayaswal, J. Bacteriol. 175:1493-1499, 1993). An open reading frame, designated lytM, of 948 bp that could encode a polypeptide of 316 amino acid residues was identified. The calculated molecular mass of the lytM gene product (34.4 kDa) corresponded to that of the autolytic activity detected (approximately 36 kDa) in the Lyt- mutant. Results deduced from amino acid sequence analysis and N-terminal amino acid sequencing data suggest that LytM is a secreted protein. The C-terminal region of the putative protein encoded by lytM showed 51% identity with the N-terminal region of the mature lysostaphin from Staphylococcus simulans and 50% identity with the N-terminal region of ALE-1 from Staphylococcus capitis EPK1. Northern blot analysis showed that lytM expresses a transcript of approximately 955 bp, as predicted from the DNA sequence. Escherichia coli clones carrying the lytM gene exhibited autolytic-activity bands of approximately 36 kDa as well as of 19 and 22 kDa in activity gels. The lytM gene was mapped to the SmaI-D fragment on the S. aureus chromosome. Mapping data and results of hybridization experiments with primers generated from gene sequences of known autolytic genes of S. aureus clearly indicate that the lytM gene is distinct from other staphylococcal autolytic genes reported to date.