[Genetic control of purine biosynthesis in the yeast Pichia methanolica. "Roman's effect" in red adenine-dependent pur6 and pur7 mutants].

Most ade1 and ade2 mutants of the yeast Pichia methanolica generate white and pink secondary colonies (SCs) on the surface of red colonies in complete medium. The formation of SCs was shown to be caused by "Roman's effect" described in Saccharomyces cerevisiae. This effect is known to result from the preferential growth in a colony of spontaneous mutations for genes controlling the first five steps of purine biosynthesis. It has been found that the ADE3, ADE5, ADE4, 7, and ADE8 loci correspond to these genes. In studies on twelve red adenine-dependent mutants, the expression of Roman's effect was shown to vary in different cultures, dependent on the original mutant ade1 and ade2 alleles. It was assumed that spontaneous mutability of each gene of purine biosynthesis ADE ade does not depend on mutations in other genes of this biosynthetic pathway. On the basis of this assumption and data from genetic analysis of mutants, it is concluded that the ade1 and ade2 mutant alleles do not determine the mutability level but affect the level of selective advantage of double mutants. This conclusion was used to estimate the lowest frequencies of spontaneous mutability of early purine genes in P. methanolica (with respect to the identified mutants). As calculated for one red mutant, frequencies of spontaneous mutations in the most mutable ADE5 gene and in the least mutable ADE8 gene of this group were 2.5 x x 10(-8) and 0.4 x 10(-8), respectively.