THE PRIMASE OF BROAD-HOST-RANGE PLASMID R1162 IS ACTIVE IN CONJUGAL TRANSFER

The broad-host-range plasmid R1162 is conjugally mobilized at high fre quency 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 (ori T) and the Mob proteins of R1162 are required, indicating that these p lasmids 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 75:271-28 8, 1989), When R751 is the mobilizing vector, the primase is not requi red for mobilization of plasmids containing cloned mob-oriT R1162 DNA. However, detectable mobilization of such plasmids by pOX38 requires b oth the primase and its cognate initiation site, oriented for synthesi s of the complement to the transferred strand, The long form of the pr imase is required for optimal transfer: R1162 replicons lacking this f orm also are not transferred detectably by pOX38 and are less well mob ilized by R751. The distance between oriT and the primase initiation s ite affects the frequency of mobilization, and this effect is polar in the direction of transfer, Our results indicate that the R1162 primas e is active in mobilization of R1162 and suggest that the MobA-linked form is an adaptation increasing its effectiveness during transfer.