Effect of 5-azacytidine on deoxyribonucleic acid methylation in Escherichia coli K12

5-Azacytidine inhibits Escherichia coli DNA(cytosine-5)methylase when added to growing cells. The time-course of recovery of methylase activity and the appearance of 5-methylcytosine in DNA following removal of the drug was studied. When E. coli K12 was treated with 5-azacytidine for 30 min, DNA (cytosine-5)methylase levels decreased to less than 10% of control levels and slowly recovered to control levels after seven generations of growth. 5-Methylcytosine synthesis in DNA remained at less than 10% of control levels for three generations after treatment and returned to control levels after six generations of growth. In contrast, beta-galactosidase levels in induced cells, which declined to 66% of control one generation after treatment, returned to control by the third generation of growth. The rate of induction of beta-galactosidase had returned to the control rate two generations after growth resumed. Since azacytidine-containing DNA inhibits DNA-cytosine methylases in vitro, the prolonged inhibition of cytosine methylation in E. coli K12 following treatment with the drug could be due to the persistence of the drug in DNA and thus inhibition of newly synthesized enzymes.