Replication of M13 oriC bacteriophages in Escherichia coli rep mutant is dependent on the cloned Escherichia coli replication origin

The involvement of the Escherichia coli rep protein in the replication of M13 chimeric deoxyribonucleic acids (DNAs) carrying the E. coli chromosomal DNA replication origin (oriC) has been examined. Previous studies indicate that the cloning of a 3,550-base-pair sequence of chromosomal DNA containing oriC into an M13 vector allows extensive replication of the M13 oriC chimeric DNA in an E. coli rep-3 mutant. We have extended these studies by preparing a 330-base-pair deletion that specifically deletes the oriC sequence in the M13 oriC DNAs, to demonstrate that the replication observed in the rep-3 host is dependent on the cloned origin. Thus, a DNA-unwinding enzyme other than the rep protein may be involved in the strand separation process accompanying replication which initiates at oriC in the M13 oriC chimeric DNAs and in the E. coli chromosome. The rep assay used for assessing the functionality of the cloned oriC is useful for analysis of any rep-independent origin of replication functional in E. coli. A direct selection for a cloned origin of replication is possible in the rep-3 recA56 host. Since the cloned origin is nonessential for propagation of the M13 chimeric phage in a rep+ host, mutations in the cloned origin may be constructed, and the mutant phage may be examined by a simple transductional analysis of the rep-3 recA56 mutant strain.