Mechanism of Action of Nalidixic Acid on <i>Escherichia coli</i> II. Inhibition of Deoxyribonucleic Acid Synthesis

Goss, William A. (Sterling-Winthrop Research Institute, Rensselaer, N.Y.), William H. Deitz, and Thomas M. Cook. Mechanism of action of nalidixic acid on Escherichia coli. II. Inhibition of deoxyribonucleic acid synthesis. J. Bacteriol. 89:1068-1074. 1965.-Nalidixic acid was shown to inhibit specifically the synthesis of deoxyribonucleic acid (DNA) in Escherichia coli. Slight effects on protein and ribonucleic acid (RNA) synthesis were observed only at higher levels of drug or after prolonged incubation. The inhibition of DNA synthesis in E. coli 15TAU, as measured by incorporation of C(14)-labeled thymine, was observed after exposure to nalidixic acid for 10 min. Inhibition of the incorporation of C(14)-labeled uracil into RNA and C(14)-labeled l-arginine into protein (21 and 28% inhibition, respectively) was observed only after 60 min of exposure. When cultures of E. coli 15TAU were exposed to 3.0 mug/ml of nalidixic acid (slightly greater than the minimal growth inhibitory concentration), the incorporation of C(14)-labeled thymidine was inhibited 30 to 40% after 90 min. Nalidixic acid at 10 mug/ml, a lethal concentration, inhibited thymidine incorporation 72% during this period. Nalidixic acid at 1.0 mug/ml had no apparent effect on the incorporation of C(14)-labeled adenine or C(14)-labeled uracil into RNA of cultures of E. coli 198, a wild-type strain. However, incorporation of both bases into DNA was strongly inhibited after 60 min of exposure (66 and 69%, respectively). Nalidixic acid inhibited DNA replication during a single round of synthesis. In contrast with "thymineless death," nalidixic acid was not lethal to E. coli 15TAU during restricted RNA and protein synthesis (i.e., in a medium containing thymine but lacking arginine and uracil). We have shown also that this chemotherapeutic agent has little effect on the synthesis of protein or RNA required to initiate DNA replication. After 75 min of inhibition, the capacity of E. coli 15TAU to synthesize DNA in a medium containing thymine, arginine, and uracil may be restored by a simple filtration and washing process, indicating that the drug is not firmly bound. These studies leave little doubt that a primary action of nalidixic acid is the inhibition of the synthesis of DNA in E. coli.