The primase of broad-host-range plasmid R1162 is active in conjugal transfer

The broad-host-range plasmid R1162 is conjugally mobilized at high frequency by the IncP-1 plasmid R751 but is poorly mobilized by pOX38, a derivative of the F factor. In both cases, the origin of transfer (oriT) and the Mob proteins of R1162 are required, indicating that these plasmids are mobilized by similar mechanisms. R1162 encodes a primase, essential for vegetative replication of the plasmid, that is made both as a separate protein and as the carboxy-terminal domain of MobA, one of the R1162 mobilization proteins (P. Scholz, V. Haring, B. Wittman-Liebold, K. Ashman, M. Bagdasarian, and E. Scherzinger, Gene 75:271-288, 1989). When R751 is the mobilizing vector, the primase is not required for mobilization of plasmids containing cloned mob-oriT R1162 DNA. However, detectable mobilization of such plasmids by pOX38 requires both the primase and its cognate initiation site, oriented for synthesis of the complement to the transferred strand. The long form of the primase is required for optimal transfer: R1162 replicons lacking this form also are not transferred detectably by pOX38 and are less well mobilized by R751. The distance between oriT and the primase initiation site affects the frequency of mobilization, and this effect is polar in the direction of transfer. Our results indicate that the R1162 primase is active in mobilization of R1162 and suggest that the MobA-linked form is an adaptation increasing its effectiveness during transfer.