THE PEPTIDOGLYCAN COMPOSITION OF A STAPHYLOCOCCUS-AUREUS MUTANT SELECTED FOR REDUCED METHICILLIN RESISTANCE

The peptidoglycan of a Tn551 mutant of Staphylococcus aureus (RUSA208) selected for reduced methicillin resistance was analyzed by reversed- phase high pressure liquid chromatography and mass spectrometry. RUSA2 08 is a member of a cluster of Tn551 mutants located on fragment A of SmaI digests but is distinct from the femA and femB class of mutants. The peptidoglycan of RUSA208 contained normal parental muropeptides bu t in diminished amounts only. The major muropeptides of RUSA208 were n ew components eluting with somewhat longer retention times from the co lumn. Amino acid analysis of these new muropeptides showed identical c ompositions to the corresponding peaks in the parental strain, but mas s spectrometry revealed increased molecular weights by the following m ass units: 1 (in monomers), 1 or 2 (in dimers), and 2 or 3 (in trimers ). These observations suggest that in RUSA208 the mutational block may be in the amidation of the stem peptide glutamate residues, resulting in the replacement of isoglutamine with free glutamic acid in one or more of the cell wall stem peptides.