pHH502, a Plasmid with IncP and Incl Characters, Loses the Latter by a Specific recA-independent Deletion Event

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Abstract
Plasmid pHH502 was isolated from Escherichia coli. It has a MW 70 .times. 106, determined resistance to tetracycline, chloramphenicol, trimethoprim, sulfonamides and mercuric chloride and was incompatible with members of IncP and IncI.alpha.. It resembled other plasmids of IncI.alpha. in that it determined pili that were morphologically and serologically I.alpha. 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 IncI.alpha. plasmids; and it was non-transferable to Proteus. In a reproducible, recA-independent event, pHH502 gave rise to pHH502-1, a plasmid of MW 40 .times. 106 lacking determinants for resistance to tetracycline and chloramphenicol and all detectable IncI.alpha. 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 P. mirabilis. It differed from other IncP plasmids in the morphology and serological type of its pili and in failing to transfer to Pseudomonas aeruginosa or Acinetobacter calcoaceticus. 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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