traY and traI are required for oriT-dependent enhanced recombination between lac-containing plasmids and λplac5

Recombination between F42lac and λplac5 is typically 20- to 50-fold more efficient than recombination between chromosomal lac and λplac5. This enhancement of recombination requires trans-acting factors located in the promoter-distal and promoter-proximal regions of the main traY-to-traI (traZ) operon. By testing the ability of deletion mutants of tra to support enhanced recombination, we have identified traY as the only essential gene from thepromoter-proximal region of tra. The possibility of a direct role for the traJ gene product has been ruled out. We also report that traI is the only gene from the promoter-distal end of the traY to traI operon that is required for recombination enhancement. Of the two proposed domains of traI, we conclude that the oriT-nicking activity is essential, whereas the helicase activity is largely dispensable. The possibility of a third traI activity is also discussed.