Boris Magasanik

Studies on the structure of ribonucleic acids.

Abstract The course of action of crystalline ribonuclease on the ribonucleic acids from yeast and from pig liver was studied with the use of chromatographic and spectroscopic procedures. The findings formed the basis for the development of a number of conceptions as to the structure of the macromolecular pentose nucleic acids. About 60 to 70 mole per cent of the nucleotides present in the initial substrates were liberated by enzymatic action as rapidly dialyzable nucleotides of low molecular weight. This fraction consisted of free cytidylic and uridylic acids (comprising a high proportion of the total pyrimidine nucleotides) and of combined purine and pyrimidine nucleotides. All nucleic acid preparations yielded a nondialyzable residue, resistant to enzymatic attack, which was found to consist to about two thirds of guanylic acid and of varying amounts of the other nucleotides. The exposure of pentose nucleic acids to weak alkali results ultimately, as is known, in their complete hydrolysis to mononucleotides. In the case of yeast ribonucleic acid, it could be shown that this rapid hydrolysis does not proceed at random, but that the several mononucleotides are released at widely differing rates, uridylic acid being the first and adenylic acid the last to be detached. The spectroscopic observations centered around a comparison of the ultraviolet extinctions of the intact nucleic acids with those of the enzymatic breakdown products and of the mononucleotide mixtures to which they gave rise on alkaline hydrolysis. At 260 mμ, the extinctions of the intact compounds and of their nondialyzable cores, produced by ribonuclease, were very considerably lower (25 to 37%) than those of the corresponding mononucleotide mixtures. The ultraviolet absorption spectra of the fractions rendered dialyzable by the enzyme showed, however, no such extinction decrease. The implications of these findings for the problems of ribonuclease specificity and ribonucleic acid structure are discussed and possible patterns of nucleotide sequence in backbone and sidechains of a branched macromolecule are considered.