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

In methicillin-resistantStaphylococcus aureus , the methicillin resistance genemecA is localized within a large chromosomal region which is absent in the methicillin-susceptibleS. aureus chromosome. The region, designatedmec DNA, is speculated to have originated from the genome of another bacterial species and become integrated into the chromosome of theS. aureus cell in the past. We report here cloning and determination of the structure of the entiremec DNA sequence from a JapaneseS. aureus strain, N315. Themec 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 ofmec DNA, and the other is situated outside themec DNA and abuts the left boundary ofmec DNA. The integration site ofmec DNA was found to be located in an open reading frame (ORF) of unknown function, designatedorfX . Clusters of antibiotic resistance genes were noted inmec DNA carried by transposon Tn554 and an integrated copy of plasmid pUB110. Both the transposon and plasmid were integrated in the proximity of themecA gene, the latter being flanked by a pair of insertion sequence IS431 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 inmec DNA. However, there was no ORF that might encodemec DNA-specific transposase or integrase proteins, indicating that themec DNA is not a transposon or a bacteriophage in nature.