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80S Ribosomes

	80S Ribosomes - Eukaryotic ribosomes are called '80S ribosomes', although actual 8 values may be lower or higher than 80S. The sedimentation coefficient is 79S-80S in fungi and 80S in mammals. The small subunit is 40S and the large subunit 60S. Eukaryotic ribosomes differ from prokaryotic ribosomes in the following respects: (I) they are considerably larger, (2) they contain a larger number of proteins (70-80 instead of 53), (3) they have four molecules of RNA instead of three, (4) their proteins and nucleic acids are larger,
(5) the RNA-protein ratio is near I : I instead of 2: 1 and (6) the first critical level of Mg++ is 0.3-0.1 micromole of MgH per micromole of phosphorus and the second critical level 0.15, compared with the values of 0.5 and 0.3 for prokaryote ribosomes. The following description of the 80S ribosome is with reference to rate liver ribosome. The 40S subunit is a curved and flattened prolate ellipsoid consisting of a 'head' (one-third) and 'body' (two-third).
It measures 230 X 140 X 115 A and it actual sedimentation coefficient is 36.9S. It contains one molecule of 188 RNA and 33 proteins. The RNA constitutes about 45% of the subunit. The proteins range in molecular weight from 11,200-31,100 with an average of 21,400. The sedimentation coefficient of the large (60S) subunit is 56.3S. It contains one molecule each of 5S, 5.8S and 288 RNA and 49 proteins. The molecular weights of the proteins range from 11,500-41,800 with an average of 21,200. RNA constitutes 59.4% of the subunit. The RNA protein ratio is 1: 1. In E.coli 5S RNA binds to proteins L5, LI8 and L25. E.coli proteins LI8 and L25 bind to 5.8S RNA from yeast (eukaryote) ribosomes but not to 58 RNA.
This suggests that eukaryote 5.8S RNA, rather than 58 RNA, is the functional equivalent of prokaryotic 58 RNA. In 80S ribosomes the first critical level of Mg++ is 0.3 to 0.1 micromoles of Mg++ per micromole of phosphorus. The second critical level is 0.15 micromoles of Mg++ per micromole of phosphorus.

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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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