Two main types of ribosomes can be found in nature (Fig. 4.3). All prokaryotic organisms, including gram-positive and gram-negative eubacteria, actinomycetes, and blue-green algae (cyanobacteria), as well as archaebacteria (archaea), contain 70S ribosomes. These ribosomes exhibit a sedimentation coefficient of about 70S; their molecular mass is approximately 2.5 x 106 daltons, and their linear dimensions (mean diameter) in a lyophilized state about 200 to 250 A; in chemical composition they are pure ribonucleoproteins, i.e. they consist of only RNA and protein. The RNA-to-protein weight ratio in them is about 2:1; correspondingly, the partial specific volume of 70S ribosomes is about 0.60 cm3/g, and buoyant density in CsCl is 1.64 g/cm3. RNA is present in the ribosomes mainly as a Mg2+, and perhaps partially as a Ca2+, salt; magnesium may account for up to 2% of the ribosomes' dry weight. Furthermore, ribosomes may contain various amounts (up to 2.5% of the dry weight) of such organic cations as spermine, spermidine, cadaverine, and putrescine. The amount of water bound in 70S ribosomes is not high, being about 1 g/g; in other words, ribosomes are rather compact unswollen particles in an aqueous medium.
The morphology of 70S ribosomes of prokaryotic organisms is almost universal, and only ribosomes of archaebacteria (archaea) have been shown to possess some differences from their eubacterial counterparts (see Chapter 5).
The cytoplasm of all eukaryotic organisms including animals, fungi, plants, and protozoans contains the somewhat larger 80S ribosomes. The molecular mass of these ribosomes is about 4 x 106 daltons, and the linear dimensions (mean diameter) are about 250 to 300 A. Like prokaryotic ribosomes they contain only two types of biopolymers - RNA and protein - but the protein content is markedly greater; the RNA-to-protein ratio in 80S ribosomes is about 1:1 by weight, the partial specific volume is about 0.65 cm3/g, and the buoyant density in CsCl is about 1.55 to 1.59 g/cm3. It is important to point out that the absolute content of both RNA and protein per particle in 80S ribosomes is markedly greater than in 70S ribosomes. The ribosomal RNA of 80S ribosomes is also bound with divalent cations, Mg2+ and Ca2+, as well as with small amounts of polyamines and diamines, e.g. spermine, spermidine, and putrescine.
Again, it should be mentioned that the morphological characteristics of all 80S ribosomes regardless of whether they have been obtained from animals, plants, or lower Eukaryotes are universal. The chloroplasts and mitochondria of eukaryotic cells, however, contain ribosomes that differ from the 80S type. The chloroplast ribosomes of higher plants belong to the true 70S type and are difficult to distinguish from the ribosomes of eubacteria and blue-green algae by the above characteristics or by more subtle molecular features. Mitochondrial ribosomes are more diverse; their properties depend on the taxonomic position of the organism from which they originate. Mitochondrial ribosomes of fungi and mammals have been studied in some detail. Mitochondrial ribosomes from fungi (Saccharomyces or Neurospora) resemble prokaryotic 70S ribosomes but are slightly larger (about 75S) and contain relatively more protein; the absolute content of ribosomal RNA seems almost identical to that found for typical 70S ribosomes. Mitochondrial ribosomes of mammals, however, are, significantly lighter than typical 70S
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