Genetic control of purine biosynthesis in yeast Pichia methanolica. The ADE5 (PUR4) gene controlling 5 '-phosphoribosylformyl glycinamidine synthetase

By comparing published and experimental data on spontaneous mutability of e arly genes controlling biosynthesis of purine nucleotides (BPN) in differen t yeast species in the system "from red to white" it was shown that the PUR 4 gene encoding 5'-phosphoribosylformyl glycinamidine synthetase (FGAM-synt hetase) (EC 6.3.5.3) is the most mutable gene in yeast Saccharomyces cerevi siae (the ADE6 gene), Schizosaccharomyces pombe (the ade3 gene), and Pichia methanolica (the ADE5 gene). This correlates with a considerably large siz e of the FGAM-synthetase polypeptide, as compared to the products of other genes belonging to this group. Study of characteristics of spontaneous muta tions in early BPN genes of P. methanolica demonstrated that the vast major ity of unstable ade5s(U) alleles (mutations with a high reversion frequency ranging from 0.2 x 10(-6) to 2 x 10(-6)) appeared solely among mutants for the ADE5 gene. Based on these results, it was assumed that there are two i ndependent mechanisms responsible for reversions of spontaneous mutations i n this gene. The DNA sequence that can compensate for the P. methanolica ad e5 mutation and probably is the structural P-ADE5 gene, was cloned from a g enomic library of P. methanolica by the ade6 mutation complementation in th e recipient S. cerevisiae strain.