Isolation and characterization of ColE1-derived plasmid copy-number mutant.

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Abstract
The plasmid pBGP120 is a ColE1 derivative that contains elements of the Escherichia coli lac operon and the Tn3 transposon. A copy-number mutant of pBGP120 was selected and isolated. In exponentially growing cultures, the copy-number mutant, pOP1, represents approximately 30% of total intracellular DNA compared to about 5% for pBGP120. Plasmid-encoded .beta.-galactosidase monomer can represent 50% of newly synthesized protein in cells carrying pOP1. pOP1 is structurally unstable in certain genetic backgrounds and under certain growth conditions, breaking down to a smaller sized plasmid that retains the DNA overproducer phenotype and the Tn3 transposon. The smaller overproducer plasmid, pOP1.DELTA.6, is generated by a continuous deletion of sequences located between 1 end of the Tn3 transposon and a site about 630 nucleotides from the EcoRI site in the .beta.-galactosidase structural gene of pOP1. pOP1.DELTA.6 retains the ColE1 origin of replication but has lost the lac promoter and operator and most of the .beta.-galactosidase structural gene. pOP1.DELTA.6 exists at approximately 210 copies/chromosome in exponentially growing cells.