MUTAGENIC SPECIFICITY OF THE BASE ANALOG 6-N-HYDROXYLAMINOPURINE IN THE URA3 GENE OF THE YEAST SACCHAROMYCES-CEREVISIAE

The mutational specificity of the base analog 6-N-hydroxyl-aminopurine (HAP) was studied in the URA3 gene of the yeast Saccharomyces cerevis iae. Twenty-nine independent HAP-induced ura3 mutations were sequenced . GC --> AT transitions were found most frequently (21 out of 29) whil e AT --> GC transitions were less abundant (five out of 29). Three GC --> TA transversions were also detected. Two interesting features of D NA context were revealed for transition mutations. One third of the tr ansitions occurred at one site within short direct imperfect repeats c onverting them to perfect repeats. A model involving complementary int eraction of imperfect repeats is proposed to explain the origin of the se mutations. Nearly all of the rest of the GC --> AT as well as the A T --> GC transitions were found in the runs of several identical base pairs, predominantly in the middle or at the 3' template nucleotide of (G)n and (A)n runs.