SUMMARY: The plasmid-encoded β-lactamase genes of six strains of were cloned and shown to be expressed in . The cloned genes were re-introduced into S. aureus via a shuttle vector, and expressed β-lactamase. However, clones containing only the small amount of DNA found necessary for expression of ampicillin resistance in E. coli did not express β-lactamase in.. Much larger pieces of DNA from the original plasmid were necessary to obtain expression in.. Some of the six strains of . synthesized β-lactamase constitutively and some released only a small proportion of the enzyme into the medium. Both these characteristics were maintained in the clones so it is concluded that they are features either of the gene itself or of the surrounding DNA. The cloned genes were sequenced and the putative amino acid sequences of the βlactamases were compared. There are several differences between the sequences and in particular one change in the N-terminal region, at a position believed to be especially important for export of proteins from the cell, is thought to have a key effect on whether or not the enzyme is found in the medium.


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