SUMMARY: Plasmid DNAs from 25 strains (14 “typical” and 11 so-called “atypical” strains) were analysed using two plasmid isolation techniques in conjunction with restriction enzyme digestion and agarose gel electrophoresis. “Atypical” strains carried two to four plasmid species, which were different from the plasmids carried by “typical” strains, and corresponded to isolate source and biotype, suggesting that plasmid content may be a useful epidemiological marker for “atypical” . The “typical” group was extremely homogeneous in plasmid content. In addition to a single large (70-145 kb) structurally related plasmid species, all strains carried a highly conserved group of three low- plasmids (pAsa1, 5.0 kb; pAsa2, 5.2 kb; pAsa3, 5.4 kb). The pAsa plasmids had different restriction maps, and directed the synthesis of four polypeptides when assayed using a transcription/translation system and [S]methionine radiolabelling. A partially conserved 6.0 kb plasmid directed synthesis of an additional polypeptide. Total cellular plasmid DNA from virulent “typical” strain A449 was also cloned in , using the vector pBR322. When analysed using a minicell expression system, 17 plasmid-encoded proteins, ranging in size from 12 to 90 kDa, were identified, including two putative exported proteins, and a chloramphenicol acetyltrans-ferase which was encoded by the 145 kb conserved, non-conjugative, plasmid.


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