The discovery of polyribosomes

By the early 1960s, evidence had accumulated that proteins were synthesized from special RNA copies of genes, named "messenger RNAs" (mRNAs), not directly from the stable RNAs found in the ribosomes of the cytoplasm. Yet, precisely how the protein chains were assembled along the RNA and, in particular, the relationship between the mRNAs and the ribosomes during protein synthesis, was obscure. In this account, I discuss how my laboratory found that multiple ribosomes traverse each mRNA, yielding the structures known as polysomes. This work led on to the first physical determination of the coding ratio, new insights into how protein chains are initiated, and an early suggestion that chloroplasts and mitochondria in eukaryotic cells might ultimately have been derived from symbiotic bacteria.     

On the fine structure of polyribosomes

Cell and Tissue Research - The ultrastructural morphology of ribosomes was studied in tissue sections of rat uterus using different fixatives (acrolein, formaldehyde, acetic acid, methanol-acetic...     

Structural aspects of eye lens polyribosomes

Epithelial cells and the outer cortex from lenses of 1-day-old calves contain polyribosomes occurring in clusters of five to ten monomers. The structure of these clusters is not affected by trypsin treatment up to 10mug./ml. of ribosomal suspension. When the whole lens is used as starting material the polyribosomal preparations are strongly contaminated with non-ribosomal material, which gives rise to ribosomal aggregates having the appearance of giant polyribosomes. These structures are sensitive to trypsin treatment but resist ribonuclease treatment. The electrophoretic pattern of the contaminating material resembles that of water-soluble lens protein.     

Association of eukaryotic aminoacyl-tRNA-synthases with polyribosomes

Upon fractionation of a mitochondria-free extract of rabbit reticulocytes into a ribosome-free extract and mono- and polyribosomes the bulk of the aminoacyl-tRNA synthetase activity was found in the fraction of mono- and polyribosomes. All the fifteen aminoacyl-tRNA synthetases were revealed, although in somewhat different quantities, in both fractions of the mitochondria-free reticulocyte extract. Aminoacyl-tRNA synthetases of the ribosome-free extract are found in two forms: RNA-binding one, and, the one having no affinity for high molecular weight RNAs. Aminoacyl-tRNA synthetases dissociated from the complexes with polyribosomes exist only in the RNA-binding form. All aminoacyl-tRNA synthetases can be removed from such complexes by an addition of 16S rRNA of E. coli, poly(U) or tRNA of rabbit reticulocytes. This testifies to labile association of aminoacyl-tRNA synthetases with the RNA-component of polyribosomes as well as to a rather nonspecific character of their interaction. After EDTA-induced dissociation of polyribosomes, the aminoacyl-tRNA synthetase activity was detected in the complex with both ribosomal subunits.     

Precipitation of polyribosomes with pepsin digested antibodies

Polyribosomes prepared from rabbit reticulocytes were precipitated with a “γG” fraction prepared from goat antiserum to rabbit hemoglobin (specific). A similar preparation from rabbit antiserum to rat hemoglobin (control) precipitated the same proportion of the polyribosomes. The non-specificity of the precipitation was significantly reduced by pre-digestion of the antibodies with pepsin. After pre-digestion of either whole antiserum or of the “γG” fraction, the precipitation of polyribosomes by the specific antibodies was increased slightly while precipitation of polyribosomes by the control antibodies was virtually eliminated.