Characterization of the F plasmid mating aggregation gene traN and of a new F transfer region locus trbE.

The product of the F plasmid transfer gene, traN, is thought to be required for the formation of stable mating aggregates during F-directed conjugation. By testing chimeric plasmids that express F transfer region segments for complementation of F lac traN mutant transfer, we mapped traN to the F transfer region between trbC and traF. Both protein and DNA sequence analysis determined the traN product to be a large, 66,000-Mr, polypeptide that undergoes signal sequence processing. The mature polypeptide was associated with outer membrane protein fractions, and a protease accessivity test confirmed that at least one portion of TraN is exposed on the cell surface. Our DNA sequence analysis also revealed that another gene, trbE, is located between traN and traF. The product of trbE was identified and shown to be a small, integral, inner membrane protein. The mating efficiency and pilus-specific phage susceptibility of a trbE::kan insertion mutant suggested that trbE is not essential for F transfer from Escherichia coli K-12 under standard mating conditions.