Appearance of newly formed mRNA and rRNA as ribonucleoprotein-particles in the cytoplasmic subribosomal fraction of pea embryos

Incorporation studies with 3H-uridine or 3H-adenosine showed that germinating pea embryos synthesize all types of poly A(+) RNA, rRNA and 4–5S RNA at the early stage of germination. After the pulse labeling for 30 min, only heterodisperse RNA and 4–5S RNA appeared in the cytoplasm as labeled RNA species. At this time the radioactivity was associated with cytoplasmic structures heavier than 80S and RNP particles of 68–70S, 52–55S, 36–38S and 20–22S which are presumed to be free mRNP particles in plants. When the pulse-labeled embryos were incubated for a further 60 min in an isotope-free medium, the labeled 17S and 25S rRNA emerged in the cytoplasm, together with labeled heterodisperse and 4–5S RNAs. More radioactivity accumulated in the regions of the polysome, 62–65S and 38–42S particles. The results of analysis of RNAs extracted from the whole cytoplasm, polysome or subribosomal fractions indicated that small subunits of newly formed ribosomes appear more rapidly in the cytoplasm than new large subunits, which accumulate for a while as free particles in the cytoplasm then are incorporated into polysomes. The actino-mycin treatment which caused preferential inhibition of rRNA synthesis reduced the accumulation of free, newly formed ribosome subunits and partially permitted detection of the presumed mRNP particles in the subribosomal region even after the chase treatment.