Studies on the induction of mitotic gene conversion by ultraviolet irradiation. I. Analysis of dose-frequency...

The UV (270-nm) dose-frequency relationship for the induction of intragenic mitotic recombination at trp 5 locus in Saccharomyces cerevisiae was non-linear. Two parameters, alpha and a, in the proposed equation for the non-linear relationship f = (at)alpha were determined so as to fit the experimental data by the method of least squares. The analysis was extended over 5 cell stages during synchronous growth. It was found that (1) parameter alpha changed from 2.02 for unbudded small cells to 1.09 for the stage where the cell had finished the division of the nucleus, and (2) parameter a changed correspondingly from 7.25-10(-4) to 0.180-10(-4) sec-1 during the same period. One interesting outcome in this analysis was the deduction of a dose-dependent nature of relative sensitivity with respect to the stage. The determination of these two parameters enabled us to calculate dose-effect relationships beyond the limits of experimental restrictions. Such an "imaginary" relationship, calculated at an extremely low dose, revealed the existence of maximal sensitivity around the DNA synthesis period. It was further shown that this maximum would easily be masked even in the moderate dose range. Thus, we conclude that the validity of single dose comparisons is diminished unless alpha is constant regardless of the cell stage. Some considerations on the proposed parameters have been made in relation to the mechanisms of the induction of gene conversion by UV.