Change of a single amino acid in the leader peptide of a staphylococcal beta-lactamase prevents the appearance of the enzyme in the medium.

A plasmid encoded beta-lactamase gene from Staphylococcus aureus (strain 3804) was cloned and sequenced. The nucleotide sequence is very similar to those obtained for other such beta-lactamase genes. The beta-lactamase has an N-terminal region characteristic of an exported protein and assay of activity shows that the enzyme is found extracellularly in S. aureus. A residue in the N-terminal region near to the postulated cleavage site was changed by site-directed mutagenesis from a serine into a proline. Comparison of beta-lactamase activity outside the cell in strains containing the cloned wild-type and mutagenised genes shows that this single amino acid completely prevents the appearance of the enzyme in the medium.