ECHINOMYCIN, A BIS-INTERCALATING AGENT, INDUCES C-!T MUTATIONS VIA CYTOSINE DEAMINATION

Echinomycin, a bis-intercalating, antitumor drug, has been studied for its ability to induce the deamination of cytosine to uracil (C --> U) in double-stranded DNA. We have employed a sensitive lacZ alpha-compl ementation reversion assay to detect G . C --> A . T mutations at a nu mber of sites in M13mp2 DNA to determine the extent to which distortio ns of DNA structure induced by echinomycin may affect C --> U rates. W hen double-stranded M13mp2 DNA with a 12-base target containing a CpG site was incubated at 37-degrees-C, the reversion frequency of the ech inomycin-treated DNA increased linearly over time, with a rate constan t 3-fold greater than DNA incubated without echinomycin. Of the 11 way s that blue pseudo-revertants can occur in the target, 96% of the obse rved revertants arose from C --> T and tandem CC --> TT transitions, w ith 78% attributable to single-base C --> T changes at three sites. Tr ansfection into ung+ cells decreased the reversion frequencies by 85% to near background levels, indicating that the increase in C --> T mut ations was due to deamination of C to U. The cytosine deamination rate constants for the entire target at pH 6.0 and 37-degrees-C were 1.2 x 10(-11) sec-1 for untreated DNA and 3.5 x 10(-11) sec-1 for echinomyc in-treated DNA. The increase in C --> T mutation rates occurred at cyt osines both proximal and distal to a CpG echinomycin-binding site. We hypothesize that this increase in deamination rate is due to a more op en or single-stranded DNA structure caused by the echinomycin: DNA int eraction.