Stimulation of Derepressed Enzyme Synthesis in Bacteria by Growth on Sublethal Concentrations of Chloramphenicol

Abstract

Culturing of Salmonella typhimurium or Escherichia coli cells in the presence of low concentrations (≤1 μg/ml) of chloramphenicol (CAP) permitted exponential growth, but at doubling times up to twice those of controls. When such cultures were subsequently starved for uracil or arginine, derepression of aspartate transcarbamylase (ATCase) or ornithine transcarbamylase, respectively, was enhanced three- to 10-fold as compared to cultures not exposed to CAP. Enhancement of β-galactosidase synthesis by prior exposure to CAP was also observed in uracil-starved E. coli cultures. Stimulation of enzyme synthesis appeared to be a specific effect of CAP; low levels of erythromycin, puromycin, sparsomycin, tetracycline, and rifampin did not show such effects. Derepression of ATCase synthesis in exponentially growing cells in the presence of CAP did not result in stimulation of enzyme synthesis by CAP. A prior history of growth of a culture in the presence of CAP was shown to be necessary for enhancement of enzyme synthesis by CAP; furthermore, continued presence of CAP in the medium during starvation was not necessary for enhanced enzyme synthesis and inhibited it in some instances. Enhanced enzyme synthesis in starving, CAP-treated cultures could be blocked by rifampin, which suggested that CAP treatment allows prolonged or more extensive messenger ribonucleic acid synthesis.

Stimulation of Enzyme Synthesis by Sublethal Concentrations of Chloramphenicol Is Not Mediated by Ribonucleotide Pools

“…CAP enhances enzyme synthesis in cultures that are starved for arginine as well as cultures starved for uracil (8). Although amino acid starvation might not be expected to lead to nucleotide depletion, stringent strains of E. coli have been reported to be restricted in their ability to take up exogenous bases during amino acid starvation (3, 6, 7).…”

“…Of a series of antibiotic inhibitors of protein synthesis that were tested, only CAP stimulated enzyme synthesis in starving cultures (8). If CAP acts to stimulate enzyme synthesis through elevation of nucleotide pools, those antibiotics that do not stimulate enzyme synthesis should fail to elevate nucleotide pools during growth and uracil starvation.…”

“…We have suggested that enhancement of enzyme synthesis by CAP in uracil-starved cultures could result from increased availability of pyrimidine nucleotides (8). Since breakdown of unstable RNA in exponentially growing cells contributes as much as 50% of the total influx into intracellular nucleotide pools (17,22) and CAP has been reported to affect both RNA synthesis and breakdown (5,11), nucleotide pools in cultures exposed to CAP were determined.…”

“…Low concentrations of the antibiotic have been shown to elevate the rate of RNA synthesis, and high concentrations Qf CAP have been shown to relieve stringent control of RNA synthesis (13). These considerations have led us to suggest that CAP stimulation of enzyme synthesis in starved cells is the consequence of altered ribonucleotide metabolism (8). In particular, we suggested that enhanced enzyme synthesis could result from increased availability of ribonucleotides from turnover of "chloramphenicol RNA" (11,16) or from alterations in the regulation of RNA synthesis.…”

“…It was shown in the accompanying paper (8) that growth of Salmonella typhimurium or Escherichia coli in the presence of low concentrations (<1 gg/ml) of chloramphenicol (CAP) substantially enhances the synthesis of repressible (or inducible) enzymes when the signal to derepress is presented under starving (nongrowing) conditions. A prior history of exposure to CAP is necessary and sufficient for maximal enhancement of synthesis, which suggests that some indirect effect of CAP on metabolism mediates increased enzyme synthesis.…”