Copolymers of Adenylic and Uridylic Acids

Although the detailed secondary structure of native deoxyribonucleic acid has been fairly well understood for some time (I), that of ribonucleic acid is only beginning to be clarified to a comparable degree. Doty et al. (2) have recently presented a tentative model for ribonucleic acid structure which is intermediate to the completely helical and completely amorphous extremes, and which consists of short helical segments which alternate with random regions. It is as yet uncertain whether the helical regions are parallel or antiparallel in character. In the former case a ribonucleic acid molecule would have to include two or more polynucleotide strands. In the latter case there exists the additional possibility that a single strand might be bent back upon itself to form hairpinlike helical regions of limited extent. I f this model proves to be correct, there remains, of course, the difficult problem of how the helical regions preserve their stability in the face of the severe steric difficulties imposed by the inevitable occurrence of extensive mismatching. In any event it is obviously desirable to approach the problem by as many avenues as possible. One possible approach is to study the properties of the biosynthetic polyribonucleotides which have recently become available. These have the definite advantage of permitting a systematic variation of composition. The present study is centered upon copolymers of adenylic and uridylic acids, which possess the further advantage of having undergone some degree of characterization in the hands of other workers (3). It was early recognized by Heppel et al. (3) that the biosynthetic copolymers of adenylic and uridylic acids produced by the polynucleotide phosphorylase of Azotobacter vinelandii possessed the C-5’-C-3’ phosphodiester linkage characteristic of all the natural and biosynthetic polynucleotides which thus far have been studied. It was also found that the linear distribution of nucleotides was essentially random in nature, and that the probability of occurrence of a sequence of adenylic or uridylic residues of a given length was apparently governed by purely statistical considerations (3). This latter feature presumably represents a departure from the case of natural ribonucleic acid, the biological activity of which is inexplicable except in terms of some specificity of sequence. iSevertheless, it might be expected that the over-all physical properties of poly AU1 would be analogous to those of native ribonucleic acid as there is no reason to suppose that the latter depend in any way upon the base sequence, in view of the similar behavior of ribonucleic acids from many sources (2). ,4n additional question also arises in the case of biosynthetic