MUTAGENICITY OF 5-BROMOURACIL AND N-6-HYDROXYADENINE STUDIED BY YEASTOLIGONUCLEOTIDE TRANSFORMATION ASSAY

The mutagenicity of 5-bromouracil (BrU) and N-6-hydroxyadenine (HA) wa s tested by means of the yeast oligonucleotide transformation procedur e. BrU-containing oligonucleotide was not mutagenic; although two muta nts (per 200 mu g oligonucleotide) were obtained, they were attributed to base insertion or base substitution at positions different from Br U. This result supports the view that BrU mutagenesis is dependent on intracellular nucleotide pool imbalance. In contrast, HA-containing ol igonucleotide was highly mutagenic; 56 mutants (per 140 mu g oligonucl eotide) were obtained. Of 21 induced mutants examined, 20 had G and on e had C at the HA position, a result indicating that HA --> G changes took place. To provide back-up evidence, we carried out a general reve rsion assay for base HA using a set of yeast tester strains, and the r esults showed that HA induces exclusively AT-to-GC and GC-to-AT transi tions. We conclude that in S. cerevisiae HA is a classic base analog m utagen, causing AT-to-GC and GC-to-AT transitions by ambiguous base pa iring. The present work has clearly demonstrated the usefulness of the oligonucleotide transformation procedure for elucidating mutagenicity of modified bases.