Effects of rifampicin, streptolydigin and actinomycin D on the replication of Col E1 plasmid DNA in Escherichia coli

We recently reported (Clewell et al., 1972) on an inhibitory effect of rifampicin on Col E1 plasmid replication. The present study represents a further characterization of this phenomenon as well as a study of the effects of two other known inhibitors of RNA synthesis, Streptolydigin and actinomycin D.During treatment of cells with chloramphenicol the colicinogenic factor E₁ (Col E1) continues to replicate for more than ton hours. During this time 4 to 5 S RNA is also synthesized. When varying concentrations of rifampicin were included during chloramphenicol treatment, inhibition of plasmid DNA synthesis correlated very closely with inhibition of cellular RNA synthesis. Similar experiments testing the effects of Streptolydigin and actinomycin D (during chloramphenicol treatment) showed that cellular RNA synthesis was at least 100 times more sensitive to these drugs than was plasmid DNA synthesis.When actively growing cells (i.e. cells not treated with chloramphenicol) were treated with a high concentration of rifampicin (250 μg/ml), chromosomal DNA synthesis continued to an extent representing about a 50% increase in DNA, while plasmid DNA synthesis appeared to stop abruptly.