Conjugal transfer of the virulence plasmid of Salmonella enterica is regulated by the leucine-responsive regulatory protein and DNA adenine methylation

Host-encoded functions that regulate the transfer operon (tra ) in the virulence plasmid of Salmonella enterica (pSLT) were identified with a genetic screen. Mutations that decreased tra operon expression mapped in the lrp gene, which encodes the leucine-responsive regulatory protein (Lrp). Reduced tra operon expression in an Lrp(-) background is caused by lowered transcription of the traJ gene, which encodes a transcriptional activator of the tra operon. Gel retardation assays indicated that Lrp binds a DNA region upstream of the traJ promoter. Deletion of the Lrp binding site resulted in lowered and Lrp-independent traJ transcription. Conjugal transfer of pSLT decreased 50-fold in a Lrp(-) background. When a FinO(-) derivative of pSLT was used, conjugal transfer from an Lrp(-) donor decreased 1000-fold. Mutations that derepressed tra operon expression mapped in dam , the gene encoding Dam methyltransferase. Expression of the tra operon and conjugal transfer remain repressed in an Lrp(-) Dam(-) background. These observations support the model that Lrp acts as a conjugation activator by promoting traJ transcription, whereas Dam methylation acts as a conjugation repressor by activating FinP RNA synthesis. This dual control of conjugal transfer may also operate in other F-like plasmids such as F and R100.