Transient transcriptional activation of the IncI1 plasmid anti-restrictiongene (ardA) and SOS inhibition gene (psiB) early in conjugating recipient bacteria

The ardA gene of the enterobacterial plasmid CoIIbP-9 acts to alleviate res triction of DNA by type I systems, while psiB inhibits induction of the bac terial SOS response. Both genes are transferred early in a round of bacteri al conjugation as part of the plasmid leading region. We report here that a rdA and psiB are transcribed transiently after their conjugative transport into the recipient cell. Transcript levels, monitored by competitive revers e transcription-polymerase chain reaction (RT-PCR) amplification of RNA tem plates, started to increase about 5 min after the initiation of conjugation in a cell population and probably before the first round of plasmid transf er was completed. Genetic evidence is given that the expression of ardA and psiB is activated when the genes enter the recipient cell on the transferr ing plasmid strand. It is proposed that these and other leading region gene s function to promote the establishment of the immigrant plasmid in the new host and are expressed by transcription from promoters active only in sing le-stranded DNA.