Copy number mutations (Cop-) of the plasmid containing the replication origin (oriC) of the Escherichia coli chromosome: lethal effect of the cop region cloned onto a high-copy-number vector on host cells.

High-copy-number mutants were isolated from an oriC plasmid. They carried insertion mutations within a region (about 470 base pairs) near the uncB gene. When a segment containing this region was cloned onto a high-copy-number plasmid, such a plasmid could be maintained as an intact form only when it was present in a lower copy number.