Genetic Analysis of the Role of the Transfer Gene, traN, of the F and R100-1 Plasmids in Mating Pair Stabilization during Conjugation

Mating pair stabilization occurs during conjugative DNA transfer whereby the donor and recipient cells form a tight junction which requires pili as well as TraN and TraG in the donor cell. The role of the outer membrane protein, TraN, during conjugative transfer was examined by introduction of a chloramphenicol resistance cassette into the traN gene on an F plasmid derivative, pOX38, to produce pOX38N1::CAT. pOX38N1::CAT was greatly reduced in its ability to transfer DNA, indicating that TraN plays a greater role in conjugation than previously thought. F and R100-1 traN were capable of complementing pOX38N1::CAT transfer equally well when wild-type recipients were used. F traN, but not R100-1 traN, supported a much lower level of transfer when there was an ompA mutation or lipopolysaccharide (LPS) deficiency in the recipient cell, suggesting receptor specificity. The R100-1 traN gene was sequenced, and the gene product was found to exhibit 82.3% overall similarity with F TraN. The differences were mainly located within a central region of the proteins (amino acids 162 to 333 of F and 162 to 348 of R100-1). Deletion analysis of F traN suggested that this central portion might be responsible for the receptor specificity displayed by TraN. TraN was not responsible for TraT-dependent surface exclusion. Thus, TraN, and not the F pilus, appears to interact with OmpA and LPS moieties during conjugation, resulting in mating pair stabilization, the first step in efficient mobilization of DNA.