Abstract
A novel ribonucleoprotein (RNP) particle showing a highly compact and characteristic structure in the electron microscope was found associated with globin and other repressed mRNA in the cytoplasm of duck, mouse and HeLa cells. This 19S complex is of extraordinary stability: dissociated by 0.5 M KCl or EDTA from the (still repressed) core globin mRNP, it can be purified on gradients containing 1% Sarkosyl, and resists (unfixed) caesium sulphate-dimethylsulphoxide density centrifugation. Its density of 1.31 g/cm3 indicates an RNP complex with a 15% RNA component. In mouse and duck it contains approximately 10 proteins in the 20 000-30 000 mol. wt. range, a few components of 50 000-70 000 mol. wt., and two specific small cytoplasmic RNAs (ScRNA) of 70-90 nucleotides. Both of these RNAs have identical 3'-terminal oligonucleotides. We propose the name 'prosome' for this ScRNP particle which somehow participates in negative control of mRNA translation, and we believe will prove to be ubiquitous to animal species.