Inhibition of plasmid DNA replication by rifampin in Salmonella pullorum

The bacterium, Salmonella pullorum, contains about 200 cytoplasmic circular DNA duplex molecules of average molecular weight (1.47 ± 0.12) × 106 daltons. Their functions are unknown. Chloramphenicol (150 μg/ml) addition to log phase cultures reduces the linear rate of plasmid synthesis four-fold. The extent of plasmid replication in the presence of chloramphenicol is sufficient to allow only one doubling of the plasmid population in six hours. Rifampin (1 μg/ml) an inhibitor of bacterial DNA-dependent RNA polymerase, inhibits plasmid synthesis in the presence but not the absence of chloramphenicol. Bacteria with a rifampin-resistant RNA polymerase replicate plasmid in the combined presence of both antibiotics.