CYCLOBUTANE THYMINE DIMERS IN A RAS PROTOONCOGENE HOT-SPOT ACTIVATE THE GENE BY POINT MUTATION

ras proto-oncogenes with a cyclobutane-type thymine photodimer (cis-sy n or trans-syn isomer) were constructed by replacement of a portion of the gene with a chemically synthesized fragment. When the genes were transfected by the calcium phosphate method into mouse NIH3T3 cells, t hey induced focus-formation, indicating that both photoproducts were m utagenic in mammalian cells. Sequence analysis of the ras gene fragmen ts derived from the transformed cells showed that the genes were activ ated by a point mutation. The mutations detected most frequently were 3'-T --> A for the cis-syn isomer and 5'-T --> A for the trans-syn iso mer. In contrast, a different trend of mutations was observed when a p rimer on a DNA template with a cis-syn dimer was extended in vitro by either DNA polymerase beta or alpha.