| Abstract: | Aza 165 and aza 238 Saccharomyces cerevisiae mutants characterized by a 2.5 times higher sensitivity of the de novo purine synthesis to the inhibitory effect of exogenous guanine, as compared with the wild type strain, have been selected by their sensitivity to 8-azaguanine. The exogenous guanine somewhat inhibits the growth and synthesis of nucleis acids in mutants, this being due in vivo neither to permeability changes of the cell membrane, nor to concentration changes of guanilic derivatives in the acid-soluble pool of yeast cells. Using cell-free extract of the strain aza 165, it has been shown that the synthesis of the first product of metabolic pathway for de novo formation of purines, phosphoribosylamine, is inhibited by GMP by 81% and only by 35% in the 15V-P4 strain of the wild type. The inhibition by other end products, IMP and AMP, is the same in both wild and mutant strains. The enhanced sensitivity of the purine synthesis to guanine in vivo is thus due to changes in regulatory properties of the key enzyme of purine nucleotide formation, phosphoribosylpyrophosphate amido-transferase (EC 2.4.2.14). This change in the regulation of purine synthesis in yeast is likely to be a mechanism to compensate the genetically controlled defect in end steps of the biosynthesis pathway, i.e. the incapability of converting guanilic derivatives to adenilic ones. However, the information concerning the regulation of PRPP-amido-transferase activity responsible for differential sensitivity to adenilic and guanilic nucleotides in yeast is not lost but only strongly repressed. |
