To assess the contribution made by mRNA-containing particles to the heterogeneity previously observed among rat liver 40S ribonucleoprotein particles, the amount of poly(A)-containing RNA in subribosomal particles was determined. RNA was labelled with orotate in vivo for 24h and then for 50min. Poly(A)-containing RNA was trapped on filters impregnated with poly(U). Very little poly(A)-containing RNA was found in conventionally prepared ribonucleoprotein particles after fractionation in sucrose. However, after preparation of ribonucleoprotein particles by sedimentation through 1 M-sucrose in the presence of 0.15M-KCl or by precipitation with Mg2+ as described by Leitin & Lerman [(1969) Biokhimiya 34, 839-849], amounts of poly(A)-containing RNA were similar to amounts of mRNA found by other workers in total ribonucleoprotein particles. Even in such preparations, less than 5% of the total rapidly labelled RNA in native subribosomal-particle fractions was mRNA. It seems that mRNA-containing particles make up only a very small part of the population of subribosomal particles in liver.

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