Chloroplast Ribosomes as Ribosomes of the Prokaryotic Type

Abstract

Summary Ribosomes have been isolated from two species of photosynthetic bacteria, five species of blue-green algae, green algae (Chlorella pyrenoidosa) and two species of higher plants — Pisum sativum and Chenopodium album. Spectral, chemical, physicochemical and electron microscopic properties of the isolated ribosomes have been studied. The RNA/protein ratio in the ribosomes of photosynthetic bacteria and blue-green algae is about 1.7 (ϱ 1.61—1.64 g/ml), and in the cytoplasmic ribosomes of Chlorella pyrenoidosa and Chenopodium album is about 1.0 (ϱ 1.55—1.56 g/ml). Ribosomes from chloroplasts and from photosynthetic bacteria and blue-green algae obtained by the same method differ in the RNA/protein ratio (ϱ of chloroplast ribosomes 1.568—1.576 g/ml). 70 S chloroplast ribosomes are much smaller than 80 S cytoplasmic ribosomes (3.0—3.1 × 106 and 4.0 × 106 daltons) and slightly larger than 70 S E. coli ribosomes (2.5 × 106 daltons). Proteins of ribosomes from pea chloroplasts and Chromatium vinosum cells have been examined by two-dimensional electrophoresis in Polyacrylamide gel. 71 protein have been found in the electrophoregram of proteins from chloroplast ribosomes. It has been shown that chloroplast ribosomes contain more proteins than Chromatium vinosum ribosomes and, as reported, E. coli ribosomes. It appears that plastid ribosomes are not identical with the ribosomes of the prokaryotic type and differ in at least the relative content and number of constituent proteins.