Studies on the transfer of IncIalpha plasmids in Escherichia coli K-12.

The studies concern the mechanism that primes the synthesis of DNA complementary to the strand of DNA that is transferred during IncIalpha plasmid-mediated conjugation. This DNA synthesis was found to be independent of the two defined priming enzymes of the host bacterium, RNA polymerase and primase. It is proposed that the synthesis is initiated by a novel priming enzyme which is specified by the plasmid and supplied by the donor parent. It was found that rifampicin, an inhibitor of RNA polymerase, has complex effects on mating bacteria. Rifampicin treatment of recipients enhanced plasmid transfer from drug-resistant donors, implying that the amount of DNA transferred is normally limited by the synthesis of a plasmid-specified product made in newly infected recipients. Enhanced transfer occurring in rifampicin-treated matings has been exploited to search for intermediates of transfer. The results show that the transferred material, isolated from recipients, is the same size as the parental plasmid. In previous studies it was claimed that transfer of IncIalpha plasmids was inhibited by rifampicin. It is shown that rifampicin-treatment of donors allowed transfer to proceed normally for about 10 min when it stopped abruptly. Cessation of transfer correlated with an abrupt disruption of conjugal DNA synthesis in donor cells. The inhibitory effect of rifampicin on Incl plasmid transfer may result from a side effect of the drug on DNA metabolism, rather than a direct result of inhibiting RNA polymerase. Previous models for IncI plasmid transfer, involving a requirement for rifampicin-sensitive synthesis of an untranslated RNA molecule in the donor, have been reevaluated. In an attempt to understand the molecular bases of these phenomena, a genetic analysis of IncIalpha plasmids was initiated.