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. 1978 Dec;75(12):5869–5873. doi: 10.1073/pnas.75.12.5869

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

D H Gelfand, H M Shepard, P H O'Farrell, B Polisky
PMCID: PMC393077  PMID: 104293

Abstract

The plasmid pBGP120 is a ColE1 derivative that contains elements of the Escherichia coli lac operon and the Tn3 transposon. We have selected and isolated a copy-number mutant of pBGP120. 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, pOP1delta6, is generated by a continuous deletion of sequences located between one end of the Tn3 transposon and a site about 630 nucleotides from the EcoRI site in the beta-galactosidase structural gene of pOP1. pOP1delta6 retains the ColE1 origin of replication but has lost the lac promotor and operator and most of the beta-galactosidase structural gene. pOP1delta6 exists at approximately 210 copies per chromosome in exponentially growing cells.

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