Direct Inhibition of Col E 1 Plasmid DNA Replication in Escherichia coli by Rifampicin

COLICINOGENIC factor E1 (Col E1) is a small bacterial plasmid (4.2×106 daltons) present in colicinogenic strains of Escherichia coli1 to the extent of about twenty-four copies per cell (Clewell and Helinski, unpublished results), which continues to replicate in the presence of high levels of chloramphenicol, a specific inhibitor of protein synthesis, although the chromosome only completes current rounds of replication and ceases (Clewell and Helinski, unpublished results). The average rate of Col E1 semiconservative replication in the absence of protein synthesis is, in certain conditions, faster than (as much as eight times) the normal rate of synthesis (Clewell, unpublished results). Replication continues for 10-15 h after the addition of chloramphenicol, resulting in nearly 3,000 copies of Col E1 DNA per cell. We are taking advantage of this system to study the effects of a number of antibiotics on DNA replication, and now report evidence that rifampicin (an active semisynthetic derivative of rifamycin B)2, an antibiotic known specifically to inhibit bacterial DNA dependent RNA polymerase3-6, has a dramatic inhibitory effect on Col E1 DNA replication.