Cloning and Nucleotide Sequence Determination of the Entire <i>mec</i> DNA of Pre-Methicillin-Resistant <i>Staphylococcus aureus</i> N315

<jats:title>ABSTRACT</jats:title> <jats:p> In methicillin-resistant <jats:italic>Staphylococcus aureus</jats:italic> , the methicillin resistance gene <jats:italic>mecA</jats:italic> is localized within a large chromosomal region which is absent in the methicillin-susceptible <jats:italic>S. aureus</jats:italic> chromosome. The region, designated <jats:italic>mec</jats:italic> DNA, is speculated to have originated from the genome of another bacterial species and become integrated into the chromosome of the <jats:italic>S. aureus</jats:italic> cell in the past. We report here cloning and determination of the structure of the entire <jats:italic>mec</jats:italic> DNA sequence from a Japanese <jats:italic>S. aureus</jats:italic> strain, N315. The <jats:italic>mec</jats:italic> DNA was found to be 51,669 bp long, including terminal inverted repeats of 27 bp and a characteristic pair of direct repeat sequences of 15 bp each: one is situated in the right extremity of <jats:italic>mec</jats:italic> DNA, and the other is situated outside the <jats:italic>mec</jats:italic> DNA and abuts the left boundary of <jats:italic>mec</jats:italic> DNA. The integration site of <jats:italic>mec</jats:italic> DNA was found to be located in an open reading frame (ORF) of unknown function, designated <jats:italic>orfX</jats:italic> . Clusters of antibiotic resistance genes were noted in <jats:italic>mec</jats:italic> DNA carried by transposon Tn <jats:italic>554</jats:italic> and an integrated copy of plasmid pUB110. Both the transposon and plasmid were integrated in the proximity of the <jats:italic>mecA</jats:italic> gene, the latter being flanked by a pair of insertion sequence IS <jats:italic>431</jats:italic> elements. Many ORFs other than those encoding antibiotic resistance were considered nonfunctional because of the acquired mutations or partial deletions found in the ORFs. Two ORFs potentially encoding novel site-specific recombinases were found in <jats:italic>mec</jats:italic> DNA. However, there was no ORF that might encode <jats:italic>mec</jats:italic> DNA-specific transposase or integrase proteins, indicating that the <jats:italic>mec</jats:italic> DNA is not a transposon or a bacteriophage in nature. </jats:p>