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In the first paper in this series (Davidson, Frazer & Hutchison, 1951) it was emphasized that in experiments on the incorporation of radioactive phosphorus (32p) into the nucleic acids and their derivatives in the cell, it was essential, in order to obtain true specific activities, to separate the nucleotide fractions in a form free from all traces of highly active contaminants. The procedure of ionophoresis of nucleotides now to be described is eminently suitable for this purpose. The separation of ribonucleotides by ionexchange chromatography on columns has been described by Cohn (1949, 1950) and by Cohn & Carter (1950), and a chromatographic separation on filter paper has been described by Magasanik, Vischer, Doniger, Elson & Chargaff (1950), by Carter (19,50), by Markham & Smith (I951a) and by Boulangeti & Montreuil (1951a, b). While these methods have much to commend them, th,ey nevertheless suffer from certain disadvantages. In the method of Magasanik et al. (1950), for example, guanylic and uridylic acids share the same position on the chromatogram, while in Carter's (1950) and Markham & Smith's (1951a) procedure, the pyrimidine nucleotides share a spot. It has therefore occurred to us that a method utilizing the ionophoretic mobility of nucleotides might prove satisfactory, and the technique of ionophoresis as originally described by Consden, Gordon & Martin (1946) has accordingly been adapted for nucleotide separation. A preliminary account of this work has already been given (Smellie & Davidson, 1951).

Phosphorus compounds in the cell. 2. The separation by ionophoresis on paper of the constituent nucleotides of ribonucleic acid