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Poly(A)‐ and nonpoly(A)‐RNA associated with rat brain microsomal fractions: In vivo labelling studies

The time course of incorporation of radiolabelled precursor into RNA associated with rat brain free polyribosomes, rough membranes, and smooth membranes was measured following a single intracranial injection of [3H] orotic acid. Polyadenylated RNAs were separated from nonpolyadenylated RNAs by affinity chromatography on oligo (dT)‐cellulose columns. Poly(A)‐RNA associated with each of the microsomal fractions became more rapidly labelled than did the nonpoly(A)‐RNA of the same fractions. While the labelling profiles of the nonpoly(A)‐RNA isolated from the polyribosomes and rough membranes are similar from one fraction to another, the specific radioactivity of the poly(A)‐RNA isolated from free polyribosomes increased much more drastically than that of the poly(A)‐RNA associated with rough membranes. The labelling profiles of RNA species isolated from smooth membranes were very different in this respect from the two ribosomal fractions. There was a lag of more than four hours before significant label appeared in the RNA associated with the smooth membrane fraction. These studies demonstrate that the different populations of brain microsomal RNA are labelled at different rates, perhaps reflecting differences in the turnover of these RNAs and differences in their function.