THE EFFECTS OF IRRADIANCE ON THE TRANSLATIONAL ACTIVITY OF CYTOPLASMIC RIBOSOMES AND ON THE COMPOSITION OF PROTEINS ASSOCIATED WITH MONORIBOSOME OR POLYRIBOSOME FROM VICIA-FABA L LEAVES

Cell-free translational activities of polysomes from Vicia faba L. lea ves and the composition of proteins associated with ribosomes from pla nts grown at 10, 50, 80, or 125 W/m2 were studied. The specific activi ty was the same for polysomes from plants grown at 50 to 125 W/m2, but was 2 to 2.5 times higher when the plants were grown at 10 W/m2. The activity also increased when the plants were transferred from 125 to 1 0 W/m2 for a short period at time and declined upon the reverse transf er. A 42 kD polypeptide was detected in polysome preparations. The acc umulation of this polypeptide is characteristic for higher irradiance, advanced leaf age, or elevated environmental temperature. This protei n closely followed the sedimentation pattern of ribosomes in sucrose g radients. RNase and Triton X-100 treatments failed to remove the 42 kD protein from the polysomes. A significant portion of this polypeptide was washed out with 0.5 M KCl, indicating that it is not an integral ribosomal protein. The possible role of this protein in translational control is discussed.