Summary: Methicillin-resistant (Mc) staphylococci contain chromosomal DNA that is absent from Mc cells. This extra DNA harbours the methicillin resistance determinant and often other resistance determinants. The region can differ substantially in structure among different isolates. We present studies on the region of a group of isolates prevalent in Australia and London. Southern hybridization analyses of a prototype Australian isolate, ANS46, and an isogenic Mc deletion mutant, ANS62, allowed the physical map of the region to be extended to 55 kb. The DNA corresponding to the deletion, which includes and resistance determinants for mercury, cadmium (Cd) and tetracycline, amounted to 41 kb. It was bounded precisely at one end by the macrolides-lincosamides-streptogramin B (MLS)-resistance transposon, Tn. Near the other end was an element with homology to Tn, ΨTn, which carried the Cd determinant. The region of an American Mc isolate, R35, was found to be virtually the same as that of ANS46, except that it lacked Tn. Another class of American Mc isolates, prevalent since 1987, differs markedly from ANS46 in region organization. However, this other American class also contains an insertion of Tn in the region, and the attachment site for this insertion was found to have significant homology to attachment sites for the Tn and ΨTn insertions in the region of the Australian strain. These results suggest possible roles of Tn and Tn-like elements in the evolutionary variation of the region.


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