A novel priming system for conjugal synthesis of an IncI alpha plasmid in recipients.

Synthesis of DNA complementary to the transferred strand of an IncI alpha plasmid has been shown previously to require DNA polymerase III. The possible involvement of the two defined priming proteins of Escherichia coli K12, RNA polymerase and primase, in initiating this conjugal DNA synthesis had been examined. Primase was inactivated using temperature-sensitive dnaG3 mutants and RNA polymerase was inhibited using rifampicin. When these two proteins were simultaneously inactivated in both parental strains, the average recipient synthesised at least one single-stranded equivalent of R144drd-3 before the rifampicin-treated donors lost the ability to transmit DNA. It is proposed that the product of a plasmid transfer gene is responsible for initiating this DNA synthesis in recipients. The results imply that this protein is supplied by the donors.