1. The conformation of RNA was found to affect its behaviour on methylated albumin–kieselguhr chromatography. The less regular the secondary structure of RNA, the more tightly it binds to the methylated albumin–kieselguhr column. 2. The presence of various denaturing agents (such as urea or perchlorate) in the medium while RNA was adsorbed on the column increased the resolving power of the technique as exemplified by the separation of rat liver rRNA into two distinct peaks. A special procedure for selective adsorption of the cytoplasmic DNA-like RNA on the preparative scale has been developed. Polyribosomal mRNA (rapidly labelled RNA formed in the presence of small doses of actinomycin D) can also be adsorbed selectively by the column. 3. A type of tissue specificity was detected in nuclear RNA from rat liver, kidney, thymus and spleen by using a modified salt and temperature gradient for the chromatographic fractionation (Lichtenstein, Piker & Shapot, 1967; Shapot, Lichtenstein & Piker, 1967). It was also found that cytoplasmic RNA from the different rat tissues contained no tenaciously bound fraction at all, whereas it constituted about 50% of the nuclear RNA. The problem of the possible biological function of the tenaciously bound fraction is discussed.

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