Summary: We have compared methicillin-resistant (Mc) isolates from Australia, the UK and the USA with regard to chromosomal inserts of the macrolides-lincosamides-streptogramin B (MLS)-resistance transposon Tn. The American isolates were known to have a distinctive Tn insert, designated insert 6, which was closely associated epidemiologically with the methicillin-resistance phenotype. Southern blots of DNA from Australian and London, UK Mc isolates were hybridized with a range of probes related to Tn. The isolates had similar or identical Tn inserts, and we consider them to be a single group, designated ‘Australondon'. Australondon isolates were compared in detail with a deletion mutant, ANS62, that had lost the methicillin-resistance determinant , plus other resistance determinants resident in the region of the chromosome, and with an American Mc isolate containing Tn insert 6. The Australondon isolates had three Tn inserts. Sequence analysis with the polymerase chain reaction showed that all of these inserts differed from classical Tn in that the 3′-terminal residues of the transposons were reverse complements of the usual GATGTA. One of the Australondon inserts, designated 6B, closely resembled Tn insert 6 in the sequence of its left flanking chromosomal DNA. This insert was found to abut the deletion from the region which results in strain ANS62. We infer that Tn insert 6B is part of the region of the chromosome in Australondon isolates, supporting the idea that insert 6 of the American isolates is also part of this chromosomal region.


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