SUMMARY: Plasmid pHH502, of molecular weight 70 x 106, determined resistance to tetracycline, chloramphenicol, trimethoprim, sulphonamides and mercuric chloride and was incompatible with members of IncP and Inclα. It resembled other plasmids of IncIα in the following properties: it determined pili that were morphologically and serologically la pili, whose production was repressed in established plasmid-carrying (R) cultures; its transfer was equally efficient in liquid or on solid medium; it exerted surface exclusion against other Inclα plasmids; it was non-transferable to . In a reproducible, -independent event, pHH502 gave rise to pHH502-1, a plasmid of molecular weight 40 x 10, lacking determinants for resistance to tetracycline and chloramphenicol and all detectable Inclα characteristics. pHH502-1 was incompatible only with IncP plasmids and resembled other IncP plasmids in determining constitutive production of rigid pili, in its surface exclusion, in transferring at greater frequency on solid than in liquid medium and in being transmissible to . It differed from other IncP plasmids in the morphology and serological type of its pili and in failing to transfer to or . Small numbers of pHH502-1 rigid pili were present on bacteria carrying pHH502. Possible mechanisms for the generation of pHH502 and pHH502-1 are discussed.


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