A Role in vivo for Penicillin‐Binding Protein‐4 of Staphylococcus aureus

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Abstract
The degree of cross-linking of the peptidoglycan of S. aureus H and mutants lacking penicillin-binding proteins 1 and 4 was studied. No major changes were observed in organisms lacking protein 1; loss of protein 4 was accompanied by a marked reduction in the degree of cross-linking and the absence of a membrane-bound model transpeptidase activity. A similar effect was achieved when cultures of the staphylococci were treated with the .beta.-lactam antibiotic cefoxitin. At low concentrations (0.05 .mu.g/ml) cefoxitin shows highest affinity for protein 4 to which it appears to bind irreversibly. Treatment of the mutant lacking protein 4 with this concentration of the antibiotic did not affect the degree of cross-linkage. The possibility that the decrease in cross-linkage was a consequence of DD-carboxypeptidase activity on peptidoglycan precursors was investigated. Although both S. aureus H and the mutants possessed such activity was insensitive to benzylpenicillin and cefoxitin and the role of this enzyme(s) in peptidoglycan biosynthesis remains unknown. In vivo, protein 4 acts as a transpeptidase involved in the secondary cross-linking of peptidoglycan. This activity is necessary to achieve the high degree of cross-linkage observed in the peptidoglycan of staphylococci.

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