FROM RNA TO DNA, WHY SO MANY RIBONUCLEOTIDE REDUCTASES

It is generally accepted that DNA appeared after RNA during the chemic al evolution of life. To synthesize DNA, deoxyribonucleotides are requ ired as building blocks. At present, these are formed from the corresp onding ribonucleotides through the enzymatic action of ribonucleotide reductases. Three classes of enzymes are present in various organisms. There is little sequence similarity among the three classes of reduct ases. However, enzymic mechanisms and the allosteric behavior of the e nzymes from various organisms are strongly conserved, suggesting that the enzymes might have evolved from a common ancestor, with the class III anaerobic Escherichia coli reductase as its closest relative.