Deoxyribonucleotide biosynthesis in yeast (Saccharomyces cerevisiae). A ribonucleotide reductase system of sufficient activity for DNA synthesis

Abstract

Ribonucleotide reductase, the central enzyme of DNA precursor biosynthesis, was isolated and characterized from baker''s yeast. The enzyme activity, measured in extracts from 3 different, exponentially growing yeast strains, is high enough to meet the substrate requirement of DNA replication, in contrast to low activities found in most other organism. In thymidylate-permeable yeast cells ribonucleotide reductase activity is stimulated under both starvation and excess of intracellular dTMP. On the other hand growth of yeast in presence of 20 mM hydroxyurea did not increase enzyme activity. Yeast ribonucleotide reductase is composed of 2 non-identical subunits, inactive separately, of which one binds to immonilized dATP. The relative MW of the holoenzyme is about 250,000. The enzyme reduces all 4 natural ribonucleoside diphosphates with comparable efficacy. GDP reduction requires dTTP as effector, ADP reduction is stimulated by dGTP, whereas pyrimidine nucleotide reduction is stimulated by any deoxyribonuclotide and ATP. Enzyme activity is independent of exogenous metal ions and is insensitive towards chelating agents. Hydroxyurea inactivates yeast ribonucleotide reductase in a slow reaction; half-inhibition (I50) is reached only at 2-6 mM hydroxyurea concentration. Up to 50% reactivation occurs spontaneously after removal of the inhibitor. In accord with previous attempts by others, extensive purification of the yeast enzyme has failed owing to its extreme instability in solution; the half-life of about 11 h could not be influenced by any protective measure.