Mechanisms of Circadian Clock Control of mRNA Translation

Summary The circadian clock, critical to human health and drug metabolism, regulates rhythmic protein production and thus cell function and metabolism. Many proteins whose levels show robust circadian rhythms are produced from mRNAs that are not rhythmic. Using the model eukaryote Neurospora crassa, we found that up to half of this circadian regulation of protein levels is through clock control of the activities of a conserved regulator of translation initiation (eIF2α), and the protein composition of translating ribosomes. We also made the surprising observation that the circadian clock controls the probability that ribosomes will read through the normal stop codon to produce proteins with carboxy-terminal extensions and potentially new functions. In addition, we found that the clock regulates the levels of tRNA synthetases that charge tRNAs with the appropriate amino acids for translation, and thus are critical for accurate protein synthesis. Over the next 5 years, we will capitalize on these findings to test the exciting hypothesis that the circadian clock controls daily changes in translation fidelity and thus protein diversity beyond what is encoded for in the genome. We will determine if clock control of ribosome composition is necessary, and which specific ribosomal proteins are required, for rhythmic stop codon readthrough. In addition, we will test if circadian clock control of binding of the co-chaperone Zuotin to ribosomes regulates daily rhythms in protein folding. We will determine the impact of circadian rhythms in methionyl-tRNA synthetase (MetRS) levels, and rhythms in the activities of kinases that phosphorylate MetRS, on three different MetRS regulatory pathways. These include translation initiation through charging of the initiator methionyl tRNA, translation elongation through charging of elongator methionyl tRNA, and misincorporation of methionine during protein synthesis through the mischarging of non-cognate tRNA. This work will significantly impact our understanding of both how a cell is different at different times of the day, and how the proteome can be more diverse than what one would predict from the genome sequence.