A collection of 346 mutations arising in the SUP4-o gene of the yeast Saccharomyces cerevisiae following treatment with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) or nitrogen mustard was analyzed by DNA sequencing. Both agents induced all possible types of base-pair substitution as well as deletions and double mutations. The base-pair changes consisted primarily of events at G·C pairs and were distributed throughout the gene. However, the distributions differed for the two drugs, and a prominent substitution hotspot was detected for nitrogen mustard. BCNU induced a substantial fraction of deletions the majority of which were recovered at a hotspot encompassing a tract of five G·C pairs. In contrast, nitrogen mustard generated relatively few deletions, but substantially more double mutations were recovered than with treatment with BCNU. Neither agent exhibited a preference for contiguous G·C sites, and more than one quarter of the mutations occurred at G·C sites, flanked by A·T pairs or at A·T pairs indicating that mutagenesis was not restricted to G·C runs. The data indicate that for BCNU and nitrogen mustard, monoadducts may play a role in mutagenesis, and site-specific mutability is influenced by factors in addition to the G·C richness of the sequence involved.

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This investigation was supported by Grant G1757 from NSERC Canada, by the University of Manitoba Research Committee of Senate, and by the Manitoba Careerstart '87 program.

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