Replication of plasmid pSC101 in Escherichia coli K12: requirement for dnaA function

Replication of pSC101 was analyzed by using DNA-DNA hybridization and alkaline sucrose gradient centrifugation. Mutants of the dnaA gene were tested for their capacity to replicate pSC101 DNA at a non-permissive temperature. Only a small amount of radioactive precursor was incorporated into pSC101 DNA in dnaA mutants at 42 degrees C whereas active incorporation into plasmid DNA took place in dnaA+ strain under the same conditions. The effect of the dnaA mutation was grater on plasmid DNA synthesis than on host chromosomal DNA synthesis. The numbers of copies of pSC101 per chromosome in wild type and dnaA strains, grown at 30 degrees C, were about 8 and 2, respectively. These results indicate that the dna A gene product is required for the replication of pSC101 DNA.