Induction of the SOS response in Escherichia coli by azidothymidine and dideoxynucleosides.

A number of nucleosides with anti-human immunodeficiency virus (HIV) activity were evaluated in two colorimetric (beta-galactosidase) assays for induction of the SOS response in Escherichia coli. 3'-Azido-3'-deoxythymidine (azidothymidine; AZT), 2',3'-dideoxyadenosine (ddA), 2',3'-dideoxyguanosine (ddG), and 2',3'-dideoxyinosine (ddI) induced cell filamentation (sulA) and prophage lambda in well-agar diffusion and liquid microsuspension assays. AZT was approximately 100 times more potent than the dideoxypurine nucleosides, inducing sulA at less than 100 ng/ml. 2',3'-Dideoxythymidine (ddT) and 2',3'-dideoxy-2',3'-didehydrothymidine (D4T) induced sulA at 100 to 1,000 micrograms/ml, while 2',3'-dideoxycytidine (ddC) weakly induced prophage lambda. Activity relationships thus were AZT greater than ddA greater than or equal to ddI greater than or equal to ddG greater than ddT = D4T greater than ddC. ddA and ddI had equivalent activities in agar diffusion assays, but different activity profiles were observed in liquid microsuspension assays. The differences may be related to drug metabolism. AZT and ddA showed marginal effects in a DNA repair (preferential toxicity) assay in which E. coli WP2 and CM871 uvrA recA lexA were used. Furthermore, none of the agents was able to preferentially inhibit Bacillus subtilis M45 recA relative to wild-type strain H17. These data suggest that AZT and the dideoxynucleosides do not cause DNA lesions that are repairable by excision repair and/or error-free postreplication repair processes. Rather, the SOS response appears to be induced by DNA chain termination leading to the inhibition of DNA replication. Bacterial assays for induction of the SOS response may be useful as simple, rapid prescreens for the discovery of new anti-HIV agents. Moreover, such assays may provide an additional parameter in the evaluation of agents with demonstrated activity against HIV and other retroviruses.