Expression of a copy number control gene (copB) of plasmid R1 is constitutive and growth rate dependent.

The copy number control gene copB from plasmid R1 was fused to the lacZ gene in vitro, resulting in expression of a fused polypeptide consisting of the first 53 amino acids of the CopB polypeptide and the beta-galactosidase polypeptide minus its first 8 amino acids. Based on measurements of specific activities of this fused protein under various conditions, it was concluded that expression of copB is gene dosage dependent, unregulated by plasmid-coded functions, and proportional to growth rate between 0.4 and 2.0 doublings per h. The rate of expression of the copB gene is surprisingly high compared with other known cases of regulatory proteins.