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Prior treatment ofEscherichia coli with nalidixic acid in nutritionally complete medium altered the subsequent pattern of deoxyribonucleic acid (DNA) synthesis normally observed in nutritionally deficient medium. Transfer ofE. coli 15 TAU to an amino acid- and pyrimidine-deficient medium usually resulted in a 40 to 50% increase in DNA content. Previous treatment with nalidixic acid caused a 200 to 300% increase in DNA content under these conditions. The extent of this DNA synthesis depended on the duration of prior exposure to nalidixic acid. The maximal rate of synthesis was obtained after a 40- to 60-min exposure to nalidixic acid and was two to three times that of the control. Theinduction of this excessive DNA synthesis was prevented by chloramphenicol or phenethyl alcohol, but thesynthesis of this DNA was only partially sensitive to these agents. WithE. coli TAU-bar, the rate of DNA synthesis, after removal of nalidixic acid, was similar to that ofE. coli 15 TAU, but the maximal amount of DNA synthesized was 180 to 185% of that initially present. Cesium chloride density gradient analysis demonstrated that DNA synthesis after removal of nalidixic acid occurs by a semiconservative mode of replication. The density distribution of this DNA was similar to that obtained after thymine starvation. These results suggest that nalidixic acid treatment may induce additional sites for DNA synthesis inE.coli .

Induction of Excessive Deoxyribonucleic Acid Synthesis in Escherichia coli by Nalidixic Acid