Size heterogeneity of the large ribosomal subunits and conservation of the small subunits in eucaryote evolution

Abstract

Ribosomes (80 S) from higher plants, sea urchins, birds and rodents were purified by zonal centrifugation in high-salt sucrose gradients and fixed with formaldehyde. The relative RNA content of the ribosomes has been calculated from the reciproacl of their buoyant density in CsCl and the partial specific volumes of RNA ( v ̄ RNA = 0.53 ) and protein ( v ̄ prot = 0.739 ). Ribosomes washed in high salt range in density from 1.581 g · cm −3 (plants and sea urchins) to 1.590–1.593 g · cm −3 (rodents and birds). The particle weight of the undissociated (high-salt washed) ribosomes has been computed from the relative RNA content (as determined by buoyant density in CsCl) and from the mol.wt of their rRNA (as determined by electrophoresis in acrylamide gel). It has been found that the 80-S ribosomes constitute a heterogeneous size-class, comprising particles having the following weights: pea seedlings and other higher plants, mol.wt = 3.90 · 10 6 ; sea urchins, mol.wt = 4.10 · 10 6 ; birds, mol.wt = 4.30 · 10 6 ; rodents, mol.wt = 4.57 · 10 6 . It has been established that any differences in size among 80-S ribosomes are exclusively accounted for by the heterogeneity of the large subunit. The functionally active subunits prepared by high-potassium treatment of “run-off” ribosomes exhibit a constant density of the small subunits ( ϱ CsCl = 1.553 ) and a range of densities of the large subunits (plants and sea urchins, ϱ CsCl = 1.595 ); chick liver, ϱ CsCl = 1.610 ; rodents, ϱ CsCl = 1.614 ). The particle wieght of the small subunits from all the eucaryotic ribosomes tested so far is consistently 1.50 · 10 6 daltons. The corresponding large subunits range in weight from a minimum value of 2.40 · 10 6 (in plants) to a maximum value of 3.00 · 10 6 (in mammals). The large subunits from sea urchins, having a weight of 2.60 · 10 6, are close neighbours of their counterparts from plants, whereas the large subunits from birds, with a weight of 2.78 · 10 6, resemble more closely those of mammalian ribosomes. It is concluded that there has been a strong evolutionary conservation of the small subunits whereas the large subunits have evolved considerably during eucaryote evolution.