Ribosomes and polyribosomes in Brassica pekinensis

Leaves of Brassica pekinensis plants contain 83-S ribosomes which occur free in the cytoplasm rather than in chloroplasts. A 68-S class of ribosomes present in 20–35% the amount of the 83-S, occurs largely if not entirely in chloroplasts. Both classes may occur as polyribosomal aggregates which can be temporarily preserved in leaf extracts using polyvinyl sulphate as a nuclease inhibitor. The major environmental influence on polyribosome levels in leaves is a diurnal cycle in which light is the predominant factor. Polyribosomes decrease in amount during the night and reach their lowest level before dawn. 2 or 3 h after sunrise 80–90% of the 83-S ribosomes may be in polyribosomes. If the natural dark period is extended for 2–3 h no polyribosomes can be detected. When such plants are exposed to sunlight a rise in polyribosomes and a drop in 83-S ribosomes can be detected within 4 min. This light-dependent increase in polyribosomes appears to be associated, at least in part, with the synthesis of new RNA.