Survival and mutagenic effects of 5-azacytidine in Escherichia coli

Survival and mutagenesis caused by 5-azacytidine was studied in Escherichia coli. Survival was partially lexA- and recA-dependent and was decreased by the presence of a DNA (cytosine-5)methyltransferase. The dcm, MspI, and EcoRII methyltransferase genes all decreased survival. There was no direct relationship between amount of methylase enzyme present and cell survival, but only plasmids containing a methylase gene sensitized cells to 5-azacytidine. Survival was not affected by uvrA, uvrB or umuCD mutations. Induction of sulA::lacZ fusions by 5-azacytidine was inhibited in strains containing elevated levels of DNA methylase. Cells resistant to 5-azacytidine when they contained a plasmid specifying the EcoRII methylase were sensitive if the plasmid specified the complete EcoRII restriction-modification system. The mechanism of cell death in these situations is therefore different.

Mutation of the rpoB gene by 5-azacytidine was studied. The mutation rate was decreased by the presence of recA and lexA mutations. Mutation in umuCD had little effect on the mutation rate. The recA430 mutation, which does not support SOS-dependent mutagenesis induced by UV light, does support 5-azacytidine induced mutagenesis. The presence of DNA (cytosine-5)methyltransferase had no effect on the mutation rate caused by 5-azacytidine treatment.

The mutagenic and lethal lesions caused by 5-azacytidine in the absence of methylase therefore differ from the lethal lesions that occur in the presence of methylase. The former could be due to the opening of the 5-azacytosine ring in DNA. Cell death in the presence of methylase could be due to tight binding of methylase to azacytosine containing DNA as well as inhibition of induction of the SOS response.