Abstract
The development of the mitochondrial enzymes fumarase and succinate dehydrogenase has been followed in Euglena cultures division-synchronized by 14-hour light periods alternating with 12-hour dark periods. The activity of both enzymes was unaltered over the light phase, doubled in early dark phase, and thereafter remained constant over the rest of the cycle. The increase in enzyme activity in early dark phase probably represented de novo enzyme synthesis because it was prevented by the addition of cycloheximide at a concentration known to inhibit protein synthesis on Euglena cytoplasmic ribosomes.When division-synchronized cultures were darkened in early light phase, a doubling of both fumarase and succinate dehydrogenase activity resulted, showing that light was repressing enzyme synthesis. The addition of acetate did not have a similar effect to darkening cultures: enzyme activity being unaltered over the light phase of the cycle. Enzyme expression was also unaffected by the addition of 3-(3,4 dichlorophenyl)-1,1-dimethylurea, a potent inhibitor of photosynthetic carbon dioxide fixation. The addition of 6-methylpurine (an inhibitor of transcription) at the beginning of the light phase inhibited enzyme increase in early dark phase, but when added at a later stage of the light phase (hour 8), increase in enzyme activity in early dark phase was unaffected. We concluded that transcription for these enzymes occurs in early light phase but light exerts a post-transcriptional control so that enzyme synthesis does not result until cells enter the dark phase of the cell cycle.