The Conversion of a Phage-induced Ribonucleic Acid to Deoxvribonucleotides

The mechanism of the biosynthesis of deoxyribose has been one of the particularly obscure problems in the biochemistry of the nucleic acids. During the past decade, considerable effort was expended in the demonstration that the deoxyribose of deoxyribonucleic acid was not derived via the condensation of glyceraldehyde a-phosphate and acetaldehyde in the presence of deoxyribose phosphate aldolsse. The properties of the enzyme-catalyzed reaction (1) suggested that the enzyme more probably participated in the degradation of the deoxypentose phosphate than in its formation. More significantly, the amount and distribution of isotope in the deoxyribose of DNA derived from selectively labeled carbohydrate precursors indicated that normal biosynthesis in cells did not involve this route (24). On the other hand, these same studies suggested that deoxyribose was derived directly from a ribose derivative or that ribose and&deoxyribose were somehow derived from the same precursor. The work of Loeb and Cohen (4) showed, for example, that the deoxyribose of all of the deoxyribonucleotides of the DNA of EschepicKu co& grown on glucose-l-W possessed essentially identical amounts of isotope, which were the ssme as that of the ribose of each ribonucleotide of the ribonucleic acid. It was found, similarly, that the deoxyribose of the pyrimidine and purine deoxyribonucleotides of the DNA of phage T6 were also derived from a common pathway (5). That deoxyribose was indeed derived from ribose derivatives has been demonstrated more directly in numerous organisms (6-12) by showing the conversion of a pyrimidine or purine ribonucleoside to the deoxyribonucleoside without cleavage of the N-glycosyl bond. These results, obtained with intact cells, have culminated in the observation by Reichard and Rutberg (13) of a conversion of a phosphorylated derivative of cytidine to that of deoxycytidine in a cell-free extract of E. coli. These reports on deoxyribose synthesis, in cells and in extracts, have not established the molecular level at which ribose is converted to deoxyribose. Exogenous nucleosides or carbohydrates were provided as precursors in the former studies; the nucleotide cytidme 5’-phosphate was employed in crude cellfree extract in the presence of Mg++, adenosine triphosphate, and reduced triphosphopyridine nucleotide (13). It was suggested by Reichard and Rutberg (13) that the reduction proceeded at the diphosphate level, although ATP was still markedly stimulatory. These data were still consistent, however, with