IS257 AND SMALL PLASMID INSERTIONS IN THE MEC REGION OF THE CHROMOSOME OF STAPHYLOCOCCUS-AUREUS

Four copies of the insertion sequence IS257 are found in the mec region of the chromosome of the Australian methicillin resistant Staphylococcus aureus (MRSA) strain ANS46, two flanking a merAmerB sequence (encoding resistance to mercurial compounds), the other two flanking an integrated copy of the plasmid pT181 (tetracycline resistance). The termini of the integrated copy of the plasmid pT181 carry a direct repeat of 8 bp of plasmid sequence, but otherwise there are no similarities in the 8 bp sequences flanking the four copies of IS257 in this strain. Integrated copies of pT181 in strains R35 (a New Jersey MRSA) and GH32 (MRSA of Greek origin) have the same terminal repeat as in ANS46, suggesting either a specific site of insertion of IS257 into the free plasmid before integration into the chromosome, or a common evolutionary lineage for these geographically diverse isolates. A different 8 bp terminal repeat of plasmid sequence is found in the chromosomally integrated copy of pUB110 (flanked by a pair of IS257s) in R155, another New Jersey MRSA. This 8 bp repeat differs from that reported previously for pUB110/IS257 inserted into the plasmid pSK41, indicating insertion of IS257 into different sites of pUB110 before integration into the chromosome or into pSK41. In the plasmid pSK1, the two outer copies of IS257 of the three associated with Tn4003 (trimethoprim resistance) are also flanked by 8 bp repeats. A single copy of IS257 adjacent to mec in the London MRSA strain N56 has the same junction sequences as those outside the pair of IS257 copies flanking pT181 in ANS46, indicating insertion of this single copy into the same point in the chromosome. The rightward junction sequence, but not the left, is identical to that reported for the copy of IS257 (also known as IS431 mec) associated with mec in strain BB270, a mec transductant of the type strain S. aureus 8325. The letfward junction sequence in BB270 is the same as the corresponding sequence in R155. None of the junction sequences of the chromosomal copies of IS257 is the same as any of the junction sequences reported for IS257 in staphylococcal plasmids. The data suggest that IS257-mediated recombinations of plasmids and transposons with the S. aureus chromosome or plasmids of the type of pSK1 and pSK41 are more recent evolutionary events than the recombinations exemplified by other plasmids and transposons carrying IS257 so far described. IS257 insertions do not appear to be site specific. © 1994 Academic Press. All rights reserved.