Mutagenicity of the cytidine analog zebularine in Escherichia coli.

We have examined the mutagenic properties of zebularine, a cytidine analog lacking the amino group at C-4 that has potential use in chemotherapy. Because the hydrate is a strong inhibitor of cytidine deaminase, its use can enhance the potency of other cytosine based compounds such as 5-azacytidine (5AzaC) and cytosine arabinoside (ara-C) that are inactivated by cytidine deaminase. Using the newly developed rpoB/Rifr system in Escherichia coli, we examined base substitution mutations caused by zebularine in the chromosomal rpoB gene. Zebularine is a potent mutagen that causes mainly G : C --> A : T transitions and favors certain hotspots. Mutations are not specific to the rpoB gene, since there is also a strong induction of mutations in the thyA gene. In the absence of mismatch repair, zebularine induces both base substitutions and frame shifts at rates well above those seen in wild-type strains treated with zebularine or in mismatch repair deficient strains without treatment. The nature of these induced mutations indicates that zebularine is stimulating the induction of increased replication errors, in addition to the targeted G : C --> A : T mutations, and that these errors are normally repaired by the mismatch repair system.