Enhancement by light of the in vitro protein synthetic activity of cytoplasmic ribosomes isolated from dark-grown maize seedlings

Abstract

The enhancement by light of the protein synthetic activity of maize leaf ribosomes was studied. The optimal Mg 2+ concentration for phenylalanine incorporation by light-ribosomes was 15 mM compared to 20 mM for dark-ribosomes. Ribosomes isolated from leaves after 3-h exposure to light were approximately twice as active as dark-ribosomes in poly(U)-directed phenylalanine incorporation at low Mg 2+ levels (10 to 15 mM), but at higher Mg 2+ levels (20 to 25 mM) this difference was not as evident. The results of puromycin release and nitrogen dissociation experiments indicated that increased levels of peptidyl-tRNA associated with light-ribosomes was at least partially responsible for the differences in activity and Mg 2+ optima of light- and dark-ribosomes. Puromycin or nitrogen treatments greatly reduced ribosome activity, especially at low Mg 2+ levels. However, the light response was not completely abolished by either of these treatments. In addition, each of these treatments increased the Mg 2+ optimum for light-ribosomes to the level optimal for dark-ribosomes. Salt-washing experiments suggested that differences in specific ribosome-associated proteins in light- and dark-ribosomes may also relate to the light enhancement effect. While salt washing did not abolish the light enhancement it did increase the level of Mg 2+ required by light-ribosomes for optimal activity to the level required by dark-ribosomes. A combination of puromycin or nitrogen treatments plus salt washing abolished the total light enhancement of protein synthesis. Thus the light enhancement of ribosome activity appears to relate to an increase both in the level of peptidyl-tRNA on ribosomes and some ribosome factor(s) which is easily dissociable by salt.