THE ENZYMATIC SYNTHESIS OF RNA: NEAREST-NEIGHBOR BASE FREQUENCIES

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Abstract
A thorough understanding of the enzymatic mechanism for the synthesis of RNA by DNA-dependent enzymes is important for obvious reasons. The implications of this reaction as a possible step in transmitting genetic information has already been mentioned. The results tabulated in this paper, together with the similar base compositions of DNA and RNA previously deter-mined, provide further evidence for the sequential assembly of ribonucleotides along a DNA template. The data are consistent with the idea that the arrangement of bases along such a template would occur by H-bonding of the appropriate base-pairs. RNA polymerase can be primed by heat-denatured DNA as well as native DNA. No definitive answer can be given to the problem whether one or both strands of DNA determine the base sequence and composition of the enzymatically synthesized RNA. In theory, according to the Watson and Crick base-pairing hypothesis, opposing strands of DNA need not have the same base composition, but they may do so on a purely statis-tical basis. The similar base ratios found for RNA and its primer DNA and the similarity in nearest-neighbor frequencies are consistent with 2 possibilities: the strands are different and both prime, or the strands are alike in compositions and base sequence and one or both may prime. In the latter case, the results offer no Information on the mechanism of priming. Although the nearest-neighbor fre- quencies determined in these experiments suggest that the sequential arrangement of nucleotldes in RNA is probably the same as that found in the primer, the above results by themselves cannot be considered unequivocal evidence for faithful replication of the base sequences found in DNA. For this reason, we have carried out CsCl density-gradient studies which will be reported in the subsequent paper22 and which provide more conclusive evidence that the RNA strands synthesized are complementary in base sequence to the deoxypoly-nucleotide primer. The term complementary is defined as a specific fit of portions of primer and product by appropriate base interactions.