Photoperiod sensitivity of the Arabidopsis circadian clock is tissue-specific

Tissue-specific functions of the circadian clock in Arabidopsis have recently been revealed. The vasculature clock shows distinctive gene expression profiles compared to the clock in other tissues under light-dark cycles. However, it has not yet been established whether the vasculature clock also shows unique gene expression patterns that correlate with temperature cycles, another important environmental cue. Here, we detected diel phase of TIMING OF CAB EXPRESSION 1 (TOC1) expression in the vasculature and whole leaf under long-day light-dark cycles and temperature cycles. We found that the vasculature clock had advanced TOC1 phase under light-dark cycles but not under temperature cycles, suggesting that the vasculature clock has lower sensitivity against temperature signals. Furthermore, the phase advancement of TOC1 was seen only under long-day condition but not under short-day condition. These results support our previous conclusion that the circadian clock in vasculature preferentially senses photoperiodic signals.     

Effects of Different PER Translational Kinetics on the Dynamics of a Core Circadian Clock Model

Living beings display self-sustained daily rhythms in multiple biological processes, which persist in the absence of external cues since they are generated by endogenous circadian clocks. The period (per) gene is a central player within the core molecular mechanism for keeping circadian time in most animals. Recently, the modulation PER translation has been reported, both in mammals and flies, suggesting that translational regulation of clock components is important for the proper clock gene expression and molecular clock performance. Because translational regulation ultimately implies changes in the kinetics of translation and, therefore, in the circadian clock dynamics, we sought to study how and to what extent the molecular clock dynamics is affected by the kinetics of PER translation. With this objective, we used a minimal mathematical model of the molecular circadian clock to qualitatively characterize the dynamical changes derived from kinetically different PER translational mechanisms. We found that the emergence of self-sustained oscillations with characteristic period, amplitude, and phase lag (time delays) between per mRNA and protein expression depends on the kinetic parameters related to PER translation. Interestingly, under certain conditions, a PER translation mechanism with saturable kinetics introduces longer time delays than a mechanism ruled by a first-order kinetics. In addition, the kinetic laws of PER translation significantly changed the sensitivity of our model to parameters related to the synthesis and degradation of per mRNA and PER degradation. Lastly, we found a set of parameters, with realistic values, for which our model reproduces some experimental results reported recently for Drosophila melanogaster and we present some predictions derived from our analysis.     

The Function of Circadian RNA‐Binding Proteins and Their cis‐Acting Elements in Microalgae

An endogenous clock regulates the temporal expression of genes/mRNAs that are involved in the circadian output pathway. In the bioluminescent dinoflagellate Gonyaulax polyedra circadian expression of the luciferin‐binding protein (LBP) is controlled at the translational level. Thereby, a clock‐controlled RNA‐binding protein, called circadian controlled translational regulator (CCTR), interacts specifically with an UG‐repeat, which is situated in the lbp 3^′ UTR. Its binding activity correlates negatively with the amount of LBP during a circadian cycle. In the green alga Chlamydomonas reinhardtii, a clock‐controlled RNA‐binding protein (CHLAMY 1) was identified, which represents an analog of the CCTR from the phylogenetically diverse alga G. polyedra. CHLAMY 1 binds specifically to the 3^′ UTRs of several mRNAs and recognizes them all via a common cis‐acting element, composed of at least seven UG‐repeats. The binding strength of CHLAMY 1 is strongest to mRNAs, whose products are key components of nitrogen metabolism resulting in arginine biosynthesis as well as of CO₂ metabolism. Since temporal activities of processes involved in nitrogen metabolism have an opposite phase than CHLAMY 1 binding activity, the protein might repress the translation of the cognate mRNAs.