Assessing the relative importance of light and the circadian clock in controlling chloroplast translation in Chlamydomonas reinhardtii

Previous work has shown that transcription of a number of chloroplast-encoded genes, including those for photosynthesis, are under circadian clock control in Chlamydomonas reinhardtii. However, some of these genes encode long-lived mRNAs that are also subject to translational control. Rates of synthesis of the major chloroplast translation products vary dramatically (10–20-fold) during light–dark (LD) cycles, peaking in the light period. To determine whether this pattern reflects circadian clock control, LD-grown cells were shifted to continuous light (LL) and chloroplast protein synthesis monitored by periodic pulse-labeling in the presence of cycloheximide; chloroplast protein synthesis in LD was also examined for comparison. The LD patterns of synthesis of the major polypeptides (including D1, D2, and the large subunit of ribulose-1,5-bisphosphate carboxylase (LS)) were similar to those obtained previously in the absence of cycloheximide. In the LL condition, rates of synthesis of the major chloroplast translation products were high throughout the period examined (∼36 h), fluctuating > 3-fold, although they were generally higher in the subjective light period. LD-grown cells were also shifted to continuous dark (DD) and chloroplast protein synthesis analyzed for ∼24 h starting from the mid-dark period. There was a gradual decline in synthesis of the major proteins during the first subjective light period, which was followed by a very small peak in synthesis around the second subjective dark → light transition. RNA blot analysis showed that the mRNAs for D1, D2 and LS were present at high levels during the period of declining translation. These results indicate that with photoautotrophic growth in LD cycles, the illumination conditions per se are more important than the clock in determining chloroplast translation, but the clock may contribute to this regulation. The advantages of controlling translation by a direct light response and transcription primarily by the circadian clock are discussed. Finally, evidence of translational control of elongation factor Tu synthesis was obtained.