Cloning and nucleotide sequence determination of the entire mec DNA of pre-methicillin-resistant Staphylococcus aureus N315

In methicillin-resistant Staphylococcus aureus, the methicillin resistance gene mecA is localized within a large chromosomal region which is absent in the methicillin-susceptible S. aureus chromosome. The region, designated m ec DNA, is speculated to have originated from the genome of another bacteri al species and become integrated into the chromosome of the S, aureus cell in the past. We report here cloning and determination of the structure of t he entire mec DNA sequence from a Japanese S, aureus strain, N315, The mec 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: o ne is situated in the right extremity of mec DNA, and the other is situated outside the mec DNA and abuts the left boundary of mec DNA. The integratio n site of mec DNA was found to be located in an open reading frame (ORF) of unknown function, designated orfX. Clusters of antibiotic resistance genes were noted in mec DNA carried by transposon Tn554 and an integrated copy o f plasmid pUB110, Both the transposon and plasmid were integrated in the pr oximity of the mecA gene, the latter being flanked by a pair of insertion s equence IS431 elements. Many ORFs other than those encoding antibiotic resi stance were considered nonfunctional because of the acquired mutations or p artial deletions found in the ORFs. Two ORFs potentially encoding novel sit e specific recombinases were found in mec DNA, However, there was no ORF th at might encode mec DNA specific transposase or integrase proteins, indicat ing that the mec DNA is not a transposon or a bacteriophage in nature.