Abstract
The buoyant density in CsCl of ribosomes from chloroplasts of the green alga Chlorella pyrenoidosa and two species of higher plants, Pisum sativum and Chenopodium album, has been studied. The buoyant density of chloroplast ribosomes has been estimated to be 1,568–1.576 g/cm 3 (47.0-45.7% protein) whereas the buoyant density of cytoplasmic ribosomes from the same plants is 1.549–1.557 g/cm 3 (50.2-49.0% protein). Under similar analytical conditions, ribosomes of prokaryotes and blue-green algae have a buoyant density of 1.61–1.64 g/cm 3 (40.0-35.0% protein). From the relative protein content it was calculated that 70S ribosomes from chloroplasts are much smaller than 80S cytoplasmic ribosomes (3.0–3.1·10 6 and 4.0·10 6 daltons) and slightly larger than 70S ribosomes from bacteria ( E. coli 2.5·10 6 daltons). It seems probable that the homologies between chloroplast ribosomes and prokaryotic ribosomes which have a number of similar physicochemical and functional properties may be more limited than previously thought.
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