Increase of methicillin resistance in Staphylococcus aureus caused by deletion of a gene whose product is homologous to lytic enzymes

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Abstract
A spontaneous high-level methicillin-resistant mutant, SRM1648, for which the MIC of methicillin is 1,600 microg/ml, was isolated on a plate containing 400 microg of the antibiotic/ml on which had been cultured the low-level methicillin-resistant Staphylococcus aureus SR17238, for which the MIC is 6.3 microg/ml. Analysis of the chromosomal DNAs of the mutant and the parental strains by the restriction landmark genomic scanning method with two-dimensional electrophoresis of restriction fragments revealed a 1.6-kb deletion in the chromosome of the mutant. The HindIII fragment of 2.5 kb containing this deleted region was cloned into a plasmid vector and introduced into the parental strain. A deletion mutant reconstructed in the presence of a low concentration of methicillin by integration and excision of the recombinant plasmid exhibited a high level of resistance (methicillin MIC, 1,600 microg/ml), confirming that the deletion had caused the elevation of the resistance level. Sequence analysis indicated that the deletion occurred in three consecutive open reading frames (ORFs). The predicted amino acid sequence of the first ORF showed high homology with both RelA and SpoT of Escherichia coli, which are involved in the synthesis and hydrolysis of guanosine 5',3'-polyphosphate, and that of the third ORF showed a relatively high homology to the lytic enzyme encoded by the lytC gene of Bacillus subtilis. We also isolated another high-level resistant mutant with a deletion within the third ORF, which suggested that inactivation of some lytic enzyme resulted in the increased resistance.