Evidence for translational regulation of chloroplast and mitochondrial biogenesis in euglena

Abstract

Changes in the rate of synthesis in vivo of 289 of the most abundant cellular polypeptides and 250 of the most abundant translatable RNAs were followed upon transferring dark grown resting Euglena to light. Pulse labelling with [ 35S] sulfate followed by 2-dimensional gel electrophoresis indicated that the relative rate of synthesis of 61 polypeptides increased and 29 decreased during the first half hour of light exposure. Changes in the rate of synthesis of other polypeptides were initially seen at 1, 4, 14, 24, 48 but not 72 h after exposure to continuous illumination. Of 18 chloroplast polypeptides found on 2-dimensional gels, the relative rate of synthesis of 5 increased during the first half hour of light exposure. The major period of elevated synthesis for 12 other chloroplast polypeptides including the large subunit of ribulose-1,5-bisphosphate carboxylase occurred between 24–72 h after exposure to continuous illumination. The synthesis rate of one chloroplast polypeptide decreased immediately upon light exposure. Of 32 mitochondrial polypeptides found on 2-dimensional gels, the relative rate of synthesis of 9 polypeptides decreased during the first half hour of light exposure and the synthesis of 12 additional polypeptides decreased by 14 h of light exposure. A transient increase in the synthesis of 4 mitochondrial polypeptides occurred during the first 14 h of light exposure, the synthesis of one was transiently increased between 24–72 h of light exposure while one mitochondrial polypeptide was synthesized at an increased rate for 72 h. Light had no effect on the rate of synthesis of 5 mitochondrial polypeptides. For most polypeptides, a sustained increase or decrease in the relative rate of synthesis was correlated with an increase or decrease in the amount of that polypeptide found in cells exposed to continuous illumination for 72 h. Changes in organelle polypeptide levels are due to stage specific changes in their rate of synthesis rather than changes in their rate of degradation. An analysis of abundant poly(A) + RNA levels by cell free translation followed by 2-dimensional gel electrophoresis failed to detect quantitative or qualitative changes in RNA abundance after 4, 12, 24 or 48 h of light exposure. Since light exposure produced major stage specific changes in the translation rate of a large number of the most abundant in vivo translation products in the absence of changes in the levels of the 250 most abundant translatable poly(A) + RNAs, translational control must play a major role in the photoregulation of the synthesis of chloroplast and mitochondrial proteins in Euglena.