Formation of delta tra F' plasmids: specific recombination at oriT

Delta tra F' plasmids can be isolated from matings between Hfr donors and recA- recipients, with selection for transfer of proximal chromosomal genes. Previous experiments indicate that F DNA from the neighborhood of the transfer origin up to the proximal junction with the chromosomal DNA is present on these plasmids, together with chromosomal segments, some of which belong to distinct size classes. We have sequenced across the novel joints contained in five delta tra FproA+ plasmids and in five delta tra FpurE+ plasmids, and we have compared these with the F sequence near oriT and with a chromosomal site near purE. The previously reported specificity in formation of some of these classes is confirmed at the nucleotide sequence level. The F DNA in nine of these novel joints extended beyond the nicking sites identified by others in lambda oriT+ bacteriophages up to a position between two sequenced oriT- mutations. Small plasmids containing these novel joints are mobilized in trans by pOX38 at frequencies less than 5 X 10(-7) times the mobilization frequencies for similar plasmids that contain oriT. The relations of these findings to the location of the nicking site at oriT are discussed.