Sensitization of Escherichia coli to actinomycin D by the arrest of protein synthesis.

Escherichia coli is generally impermeable to charged molecules for which it does not possess specific transport mechanisms (2). This permeability barrier is thought to be the cause of the resistance of E. coli to actinomycin, since this antibiotic inhibits deoxyribonucleic acid (DNA)dependent ribonucleic acid (RNA) synthesis in in vitro extracts prepared from E. coli, but has no effect on RNA synthesis in growing cells even at a concentration of 100 Atglml (1). This view is supported by the demonstration (3) that mild treatment with ethylenediaminetetraacetic acid (EDTA), which causes a release of components of cell wall lipopolysaccharide, renders cells of E. coil sensitive to actinomycin. Mutant strains of E. coli which are sensitive to actinomycin without pretreatment with EDTA have been isolated (5); it was concluded that these strains have a more permeable cell wall. In the present report, we describe another condition under which E. coli becomes sensitive to actinomycin, namely, when RNA synthesis proceeds in the absence of protein synthesis. One way of achieving this condition is by amino acid starvation of an E. coli strain possessing relaxed control of the RNA synthesis (RCrel) phenotype (6). Residual RNA synthesis during arginine starvation of an RCrel strain was actinomycin-sensitive (Fig. 1). Actinomycin had no effect on RNA synthesis in the exponentially growing, arginine-supplemented culture, but it inhibited residual RNA synthesis in the arginine-starved cultures; a concentration of 50 ,ug/ml caused a greater inhibition of residual RNA synthesis than a concentration of 10 ,ug/ml. It appears that it takes at least 30 min after the onset of amino acid starvation for the inhibition of RNA synthesis to manifest itself. Until that time, the kinetics of incorporation of "4C-uracil into trichloroacetic acid-precipitable material in the presence or in the absence of actinomycin were similar. Figure 1 also presents the results of an experiment similar to that just described, except that 14C-uracil was added 45