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70S Ribosomes

	70S Ribosomes - As mentioned previously, 70S ribosomes are found in prokaryotes (bacteria and blue green 'algae': the latter are now included in the bacteria as the Cyanobacteria). The sedimentation coefficient of prokaryote ribosomes varies from 64S to 72S, the average being 69S. The E. coli ribosome will now be described as an example of the 70S ribosome, X-ray diffraction studies reveal the dimensions of the ribosome to be 170x230x250A (Vasiliev, 1971).
The 70S ribosome consists of two subunits, a small 30S subunit and a large 50S subunit. The 308 subunits are oblate ellipsoids. Hydrodynamic and X-ray scattering techniques reveal their dimensions to be 220 x 220 k 55A, while according to electron microscopic examinations the dimensions are80x 100x 190A (Kurland. 1979). The same two techniques give the dimensions of the 508 subunit as 130 X 170 x 260A and 160 x 200 x 230A, respectively. The 308 subunit was reported to contain 21 different proteins designated as 81-821, the '8' standing for 'small'. The 508 subunit was reported to contain 34 proteins designated as Lt- L34 (the 'L' standing for 'large').
Thus the ribosome was thought to consist of 55 different proteins. It was later shown that protein 820 is identical to L26. Thus what were originally thought to be two proteins are in fact one. Protein L8 was shown to be an aggregate of proteins L7, Ll2 and LIO. Thus the prokaryote ribosome consists of 53 different proteins (Wool. 1979). Three classes of RNA have been found in 708 ribosomes, 168, 238 and 58. The 308 subunit contains 168 ribosomal RNA which has a molecular weight of about 550,000 and contains 1,600 nucleotides.
The 50S subunit contains 238 and 58 ribosomal RNA with molecular weights of 1,100,000 and 40.000, respectively. 238 RNA contains 3,200 nucleotides, and 58 RNA 120 nucleotides. The RNA and proteins are intertwined in the two subunits of the ribosome. The 70S E. coli ribosomes contain 63% RNA and 37% proteins. The RNA protein ratio is therefore 2: 1. Bacterial ribosomes may constitute up to 40% of the dry weight of the cell and 90% of its RNA. Metal ions, chiefly magnesium ions, play an important role in holding the two subunits together, and, also in maintaining the structure of the two subunits. There are two critical levels of MgH content. Below a certain level of Mg++ the two subunits of the ribosome separate.
	This level is called the first critical level, and is 0.5 micro mole of Mg++ per micromole of phosphorus in the case of the 70S ribosome. The dissociation of the subunits is reversible and the two subunits can reassociate on raising the Mg++ level. Further lowering of the MgH content below a certain level results in the breaking up of the subunits. This level is called the second critical level, and is 0.3 micromole of MgH per micro mole of phosphorus.

Thus the first critical level is for the association of the subunits, while the second critical level is for ensuring integrity of the subunits. A stabilizing role is also attributed to Ca++, Mn++ and Co++.

The 30S and 50S subunits of the ribosome have different binding properties. In E. coli the 30S subunit binds mRNA to form a 30SmRNA complex before being attached to the 50S subunit. The 50S subunit, however, cannot bind mRNA if the 30S subunit is not present.

The segment of mRNA binding to the 30S subunit is about 27 nucleotides in length. This is indicated by the following experiment. Ribosomes of E. coli were first bound to poly (U) (an RNA chain consisting of uridine phosphate nucleotides only).

They were then treated with the enzyme ribonuclease which breaks down poly (U) into its individual nucleotides. The ribosomes were purified and the ribonuclease removed. They were found to contain segments of poly (U) 184A in length. RNA has a translation of 6.8A per base in the extended chain form.

Thus the segments of poly (U) would contain approximately 27 residues. The segments are the parts of the poly (U) chain protected by ribosomes against ribonuclease digestion.

Each 70S ribosome has two binding sites for tRNA, the amino acyl (A site) or acceptor site and the peptidyl (P site) or donor site. The A site receives the tRNA-amino acid complex and the P site binds the growing polypeptidyl-tRNA. The A site is in the middle of the protected mRNA fragment.

The mRNA passes through a tunnel between the two subunits. Between the two subunits is a groove or channel through which the newly formed polypeptide chain comes out.

This channel protects the polypeptide chain from the action of enzymes the protected part of the chain is estimated to contain 35-39 amino acid residues and is probably in the extended configuration.

Assuming a spacing of 3.6A per amino acid residue in such a state the protected part of the chain would have a length of 140A. The tRNA molecule to which the polypeptide chain is attached is about 80A long.

Thus the total length of the tRNA plus the protected part of the polypeptide chain would be about 220A. This is about the observed diameter of the larger subunit.

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Ribosomes of Chloroplasts Sedimentation Coefficients of Chloroplast and Cytoplasmic ribosomes and their Subunits in Plants RNA Content of Chloroplast Chloroplast and Cytoplasmic Ribosomes Homology Between Chloroplast and Bacterial Ribosomes Ribosomal RNA Interactions Polyribosome Biogenesis of Ribosomes Functions of Ribosomes

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