Enzymatic Synthesis of Ribonucleic Acid

The discovery of enzymes which catalyze the deoxyribonucleic acid-dependent synthesis of ribopolynucleotides (l-6) stimulated an immediate inquiry into the role played by DNA in thisunique reaction. The speculation that DNA might serve as a template for the polymerization of ribonucleotides into a complementary ribonucleic acid molecule was confirmed by studies from a number of different groups (7-15). The evidence for this may be summarized as follows: (a) when different DNAs (either natural or synthetic) which varied widely in their respective guanine, adenine, cytosine, and thymine content were used, the base composition of the RNA product always resembled that of the particular primer used; (b) the nearest neighbor frequencies for the 16 possible base pairs in the RNA formed were remarkably similar to the frequencies of the same base pairs in the DNA4 primer; and (c) specific RNA-DNA hybrids could be formed only by annealing the purified RNA product with the DNA which served to direct its synthesis. Evidence has now been provided, from three independent sources, that the reaction in z&o results in the formation of RNA molecules which are complementary to both DNA strands (1215). Furthermore, the synthesized RNA is self-complementary, and is unique among ribopolynucleotides in that it may form a DNA-like highly ordered secondary structure (13). This information is included in the scheme for RNA synthesis (Fig. 1). At the present time, the question of whether one or both strands of DNA may serve to prime RNA synthesis in z&o is not resolved. Although the studies in vitro on RNA polymerase have elucidated the function of the primer and some of the physicochemical properties of the product, very little is known about the polymerization mechanism itself. A study of the properties of the reaction is important as an initial step in understanding the nature of this complex problem. Two separate DNA-primed reactions have been detected with RNA polymerase: (a) in the presence of all four ribonucleoside triphosphates, an RNA product is formed which is complementary in base composition and sequence to its DNA primer (4NTP reaction), and (a) in the presence of a single triphosphate, a ribopolynucleotide forms which contains only a single base