Hybrid Monomers Research Articles

Large and small subunits purified with high potassium from the “run-off” ribosomes of plants and mammals (whole pea seedlings, liver of rodents) can combine in low-salt buffer to form two hybrid particles: one containing the 56-S subunits from pea seedlings and the 36-S subunits from liver, and the other containing the 56-S subunits from liver and the 36-S subunits from pea seedlings. The two heterologous combinations are synthetically active when tested in a poly(U)-programmed, polyphenylalanine-synthesizing system. Following incubation in conditions of protein synthesis, the hybrid monomers carrying nascent polyphenylalanine chains remain stable at relatively high K +/Mg 2+ ratios; by contrast, the subunit “couples” not participating in protein synthesis dissociate again into their component particles. The translational efficiency of the hybrid monomers is essentially the same as that of the re-associated particles. However, the homologous and heterologous combinations containing the 36-S fraction from pea seedlings give rise to less monomers than those involving 36-S particles from both mouse liver and rat liver. It has been established that (a) close to 30 % of the small subunits from pea seedlings contain fragmented 18-S RNA which sediments at about 16 S, and (b) the 36-S fraction from plant ribosomes contains a certain proportion of faulty particles which give rise to monomers sedimenting at a slightly slower rate than the true (80-S) monomers, when combined with “healthy” large subunits (from either pea seedlings or mouse liver). The results are taken to indicate that (a) not only the size (1.50 · 10 6 daltons), but also the conformation of the small subunits has been largely conserved throughout eucaryote evolution, and (b) any change in size of the large subunits (from 2.40 · 10 6 to 3.00 · 10 6 daltons) must have involved regions of the ribonucleoprotein particle which are not crucial for subunit association.

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