Investigation Of The Mechanism Of F Plasmid Epidemic Spread: The Role Of The Strong Tram Promoter

The phenomenon of epidemic spread of a repressed F plasmid was investigated at the transcriptional level using quantitative PCR and northern blot analyses to demonstrate that readthrough transcription from the traM promoter temporarily derepresses the F transfer region. This transient derepression is thought to last for approximately six generations and ensures that all recipient cells are converted to F+ status before repression is re‐established. The F transfer region has two monocistronic operons for traM and traJ before the 33 kb transfer (tra) operon. TraM mediates the interaction between the nucleoprotein complex called the relaxosome and the inner membrane coupling protein that actively pumps the DNA into the recipient cell. TraJ is the positive activator of the transfer operon and is repressed by the antisense RNA fertility inhibition system called FinOP. The very strong traM promoter (7500 MU from a single copy plasmid) is normally autoregulated by TraM itself. We hypothesized that the traM promoter is unusually strong in order to increase the levels of traJ transcript in a new transconjugant, thereby escaping FinOP repression long enough to ensure that the tra operon is expressed. We developed a PCR‐based assay to detect transcripts that cross the traM‐traJ intergenic region in mating bacterial cultures. We also show that artificially increasing the levels of TraM in actively growing recipient cells promotes epidemic spread. This suggests that the presence of extra TraM does not immediately repress the traM promoter but that a number of factors including H‐NS, a known silencer of the F transfer region, must be considered before repression can be established.