title: Elevated intracellular dCTP levels reduce the induction of GC-->AT transitions in yeast by ethyl methanesulfonate or N-methyl-N'-nitro-N- nitrosoguanidine but increase alkylation-induced GC-->CG transversions.
abstract: The effect of an increased intracellular dCTP:dTTP ratio on the specificities of ethyl methanesulfonate (EMS) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) mutagenesis was examined in the yeast Saccharomyces cerevisiae. To do so, we used a dCMP deaminase-deficient (dcd1) strain having a dCTP:dTTP ratio > 77-fold larger than its isogenic wild-type parent under the treatment conditions employed. This DNA precursor imbalance lowered the frequencies of EMS- or MNNG-induced SUP4-o mutations by 75 or 45%, respectively, relative to the corresponding values for the wild-type strain. A total of 405 SUP4-o mutations produced by the alkylating agents in the dcd1 background were characterized by DNA sequencing and the mutational spectra were compared to those for 399 mutations induced in the wild-type parent and 207 mutations that arose spontaneously in the dcd1 strain. Unexpectedly, the frequencies of EMS- and MNNG-induced GC-->AT transitions in the dcd1 strain were found to be reduced by 93 and 68%, respectively, considerably more than the decreases for the overall SUP4-o mutation frequencies. The differences were due mainly to substantial increases in the frequencies of GC-->CG transversions. Although these events were the predominant type of spontaneous substitution in the dcd1 strain, they were more frequent after alkylation treatment and were distributed differently than the spontaneous GC-->CG transversions. Preferences for the EMS- or MNNG-induced GC-->AT transitions to occur at GC sites having the guanine located on the transcribed strand or preceded by a 5' purine, respectively, also were diminished in the dcd1 strain.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Microbiology Department, University of Manitoba, Winnipeg, Canada.
authors.forename: S E
authors.initials: SE
authors.lastname: Kohalmi
authors.forename: H M
authors.initials: HM
authors.lastname: Roche
authors.forename: B A
authors.initials: BA
authors.lastname: Kunz
mesh_terms: D000478:Alkylation; D001483:Base Sequence; D004252:DNA Mutational Analysis; D004260:DNA Repair; D004261:DNA Replication / Q000187:drug effects*; D004271:DNA, Fungal / Q000096:biosynthesis / Q000235:genetics; D003845:Deoxycytosine Nucleotides / Q000378:metabolism*; D005020:Ethyl Methanesulfonate / Q000633:toxicity*; D005800:Genes, Fungal; D016153:Genes, Suppressor; D006147:Guanine / Q000031:analogs & derivatives; D008769:Methylnitronitrosoguanidine / Q000633:toxicity*; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009153:Mutagens; D009692:Nucleic Acid Heteroduplexes; D017354:Point Mutation / Q000235:genetics*; D012441:Saccharomyces cerevisiae / Q000187:drug effects / Q000235:genetics; D013941:Thymine / Q000378:metabolism; D013942:Thymine Nucleotides / Q000378:metabolism
chemical_list: D004271:DNA, Fungal; D003845:Deoxycytosine Nucleotides; D009153:Mutagens; D009692:Nucleic Acid Heteroduplexes; D013942:Thymine Nucleotides; D008769:Methylnitronitrosoguanidine; C024107:2'-deoxycytidine 5'-triphosphate; D006147:Guanine; D005020:Ethyl Methanesulfonate; C024157:thymidine 5'-triphosphate; D013941:Thymine