Studies on soluble ribonucleic acid of rabbit liver. Terminal groups and nucleotide composition.

The discovery that soluble ribonucleic acid can function as an acceptor of activated amino acids and as an intermediate in the transfer of activated amino acids into microsomal protein (1) suggests that this nucleic acid performs a key function in the process of protein biosynthesis (2). Structural and functional studies have shown that soluble ribonucleic acid is quite homogeneous with respect to sedimentation characteristics and chain length (3, 4), and that it consists of a number of molecular types, each specific for a particular amino acid (5, 6). Every soluble ribonucleic acid molecule is thought to terminate at one end with the sequence, . . . CpCpA (7), to which the activated amino acids become linked (8)) and the opposite end of every molecule is terminated by pGp. . . (9, 10). These observations suggest that the structural basis for the specific amino acid acceptor activity and the specific transfer activity of an individual molecule resides in elements of the base sequence or composition (or both) between the pGp . . . and . . . CpCpA ends of the molecule. Although a number of methods have been described for preparing S-RNA’ enriched in acceptor activity for a particular amino acid (ll-18), no method is yet available for preparing, in relatively large quantities, S-RNA specific for a single amino acid and free of partially degraded polynucleotide chains. We have shown, however, that investigation of base sequence frequencies in unfractionated S-RNA is useful to elucidate features of the structure of the average molecule (19). When substantial quantities of highly purified S-RNA specific for a single amino acid become available, it will be possible, by application of the methods to be described in this paper, to study the relationship of the base sequence to the biological specificity of the molecule. This paper reports studies of base sequence frequencies of unfractionated S-RNA, revealed by the use of two specific ribonucleases. Taka-Diastase T1 ribonuclease specifically hydrolyzes the internucleotide bonds of RNA between 3’-guanylic acid and the 5’-hydroxyl groups of adjacent nucleotides (ZO), but it spares the bonds adjacent to the 2dimethylamino-6hydroxypurine and 1-methylguanine nucleotides (21). Pancreatic ribonuclease specifically hydrolyzes the bonds between 3’-pyrimidine nucleotides and adjacent nucleotides (22, 23). As illustrated in Fig. la, the products of hydrolysis of S-RNA by