Mutational specificity of thymine nucleotide depletion in yeast
Kohalmi, S.E.; Kunz, B.A.
Mutation Research 289(1): 73-81
1993
ISSN/ISBN: 0027-5107 PMID: 7689166 Document Number: 420679
Relative to normal growth conditions for a wild-type strain of the yeast Saccharomyces cerevisiae, withholding thymidylate (dTMP) severely diminished the dTTP pool but elevated the dATP, dGTP and dCTP levels (120-, 8.5- and 3.6-fold, respectively) in an isogenic dTMP auxtroph. This treatment also increased the frequency of mutations in a tRNA gene (SUP4-o) b 15-fold. Single base-pair events accounted for 97% of the 89 SUP4-o mutations characterized by DNA sequencing and the ratio of transversions to transitions was 3-fold greater than that for spontaneous substitutions in the wild-type strain. This difference was due to decreases in the fractions of transitions and an increase in the proportion of A cntdot T fwdarw T cntdot A transversions. The largest increases in mutation frequency were observed for transversions at A cntdot T pairs, consistent with dATP and dGTP being incorporated in place of dTTP during DNA replication. Similarly, misinsertion of dATP and dGTP could have promoted the single base-pair deletions and insertion detected. Analysis of the distributions of substitutions indicated no preference for dATP misinsertion to occur at sites flanked by a specific 5' or 3' base or on the transcribed for nontranscribed strands. However, the presence of mutational hotspots and site-specific variations in the substitution frequencies implied a role for DNA sequence context in the mutational specificity of dTTP depletion.