Conservation of a common 'backbone' in the genetic organization of the IncP plasmids RP4 and R751.

RP4 and R751 are related selftransmissible broad host range plasmids belonging to the IncP subgroups a and P respectively (1). A composite restriction map of plasmid R751 for clea-vage sites of SphI and Sall is presented. All sites including those characterized by others (2,3) were adjusted to each other. The maps are calibrated in kilobase coordinates. Although recognition sites for restriction endonucleases appear to be clustered mainly in two regiong of both plasmids, a relationship is not obvious because sites do not match. However, heteroduplex studies (4) as well as DNA hybridization experiments (3), reveal a certain degree of homology. The genetic loci for oriT (origin of transfer), pri (DNA primase), and dfr, Tpr (dihydrofolate reductase) have been determined by molecular cloning of SphI fragments into the vector plasmid pBR329. Including data on oriV (5,6) the R751 physical and genetic map is presented in a version which readily demonstrates striking similarities in the genetic organization as compared to RP4 (7). The relative order of oriV, rep (trfA), pri and oriT on both plasmids indicates the evolutionary conservation of the arrangement of essential replication and conjugation functions.