Effect of the bacterial growth rate on replication control of plasmid pBR322 in Escherichia coli

Abstract

The concentration of plasmid pBR322, of its replication inhibitor, RNAI, and preprimer, RNAII, were observed in E. coli as functions of the bacterial growth rate. At growth rates between 0.6 and 2.5 doubling/h, the copy number (number of plasmids per genome equivalent of chromosomal DNA) decreased from 32 to 15, the number of plasmids per cell increased fro, 39 to 55, and the plasmid concentration decreased from 4.6 to 1.1x10¹⁰ plasmids per OD₄₆₀ unit of cell mass. The concentrations of RNAI and RNAII also decreased with incrasing growth rate, but differently, such that their ratio, RNAI/RNAII, increased. In glycerol minimal medium both RNAI and RNAII had the same halflife, 0.55 min, and were synthesized at a ratio of about 3 RNAI transcripts per every RNAII transcript. These results were interpreted on the basis of the negative control model and suggest that the activities of the RNAI and RNAII promoters, and the efficiency with which RNAI inhibits plasmid replication, are controlled by the growth rate.