Rapidly labelled ribonucleic acid in chicken embryos: Evidence for heterogeneous ribonucleoprotein particles

Abstract

The incorporation of [ 3H]uridine into ribonucleoprotein particles was investigated. 12-day-old chicken embryos prelabelled with [ 14C]uridine for 24 h were injected intravenously with [ 3H]uridine and incubated for 15, 30 and 60 min. Analysis of [ 14C]- and [ 3H]uridine double label by sucrose and CsCl density gradients revealed rapidly labelled ribonucleoprotein particles which do not represent ribosomal material but must be considered as heterogeneous ribonucleoprotein particles. After 15 min of incubation only nonribosomal material appears in the cytoplasm, after 30 min in addition to these particles labelled 18-S RNA has already appeared in the 40-S ribosomal subunit and after 60 min the 28-S RNA of the 60-S ribosomal subunit in the cytoplasm is also labelled. The rapidly labelled nonribosomal particles were found not only in the 40–100-S region of sucrose gradients but also in the polysomal region. After 30 min and 60 min a part of the rapidly labelled nonribosomal particles is incorporated into polyribosomes. After 30 min 40% and after 60 min 60% are found in polysomes. The free non-ribosomal particles are resistant to EDTA treatment and band in CsCl gradients at ρ = 1.47 g/cm 3. This indicates a higher protein RNA ratio than found in ribosomal material. Heterogeneous ribonucleoprotein particles are also extracted from nuclei. Most of rapidly labelled RNA in nuclei is found, however, in very large structures probably representing chromatin.