A colI-specified product, synthesized in newly infected recipients, limits the amount of DNA transferred during conjugation of Escherichia coli K-12.

The amount of ColI DNA transferred between mating cells of Escherichia coli K-12 increased about fourfold when rifampin-resistant donors were mated with sensitive recipients in the presence of the drug. Conjugational synthesis of ColI in dnaB recipients, shown primarily to reflect conversion of the transferred DNA into double-stranded material, was also enhanced when the recipients were treated with either rifampin or streptomycin. It is suggested that the amount of ColI transfer is normally limited by the synthesis of one or more proteins in the newly infected recipients. The protein is thought to be plasmid-specified because rifampin also quadrupled transfer to UV-irradiated recipients which were deficient in the transcription of the resident DNA. Successive strands of ColI appear to be transferred discontinuously, because the transferred DNA accumulated in normal and rifampin-treated recipients in the form of circular and linear monomeric units. Although rifampin treatment of recipients also increased transfer of a second Ialpha plasmid, R144drd-3, by about four times, the drug failed to cause a substantial increase of Flac transfer in comparable matings.