REPLICATION OF A PLASMID LACKING THE NORMAL SITE FOR INITIATION OF ONE STRAND

The origin of replication of the plasmid R1162 contains an initiation site for the synthesis of each DNA strand. When one of these sites (or iL) is deleted, synthesis on the corresponding strand is no longer ini tiated efficiently in vitro by the R1162-encoded replication proteins, and the plasmid is no longer stably maintained in the cell. However, in vivo the two strands of the plasmid duplex molecule are active at a similar level as templates for DNA synthesis, and newly synthesized c opies of each strand are incorporated into daughter molecules at a sim ilar rate. No secondary, strong initiation sites on the Delta oriL, st rand were detected in the region of the origin. The Delta oril plasmid induces the SOS response, and this is important for plasmid maintenan ce even in a recombination proficient strain. Our results indicate tha t an SOS-induced host system can maintain an R1162 derivative lacking one of its initiation sites.