Light Response of the Circadian Waves of the APRR1/TOC1 Quintet: When Does the Quintet Start Singing Rhythmically in Arabidopsis?

Abstract

We previously identified a novel class of proteins, named A:rabidopsis pseudo-response regulators (APRRs), each of which (APRR1/TOC1, APRR3, APRR5, APRR7, APRR9) has an intriguing structural design containing an N-terminal pseudo-receiver domain and a C-terminal CONSTANS motif. Expression of these APRR1/TOC1 family members is under the control of a coordinate circadian rhythm at the level of transcription such that the APRR-mRNAs start accumulating sequentially after dawn with 2 to 3 h intervals in the order of APRR9-->APRR7-->APRR5-->APRR3-->APRR1/TOC1 in a given 24 h photo-period. Based on these data, we previously proposed that these sequential and rhythmic events of transcription, termed 'circadian waves of APRR1/TOC1 quintet', may be a basis of a presumed Arabidopsis biological clock (named 'bar code clock') [Matsushika et al. (2000) Plant and Cell Physiol. 41: 1002]. Here we further characterized the event of circadian waves, by demonstrating that certain light stimuli are crucial determinants to induce the robust circadian waves, and accordingly, the first-boosted and light-induced APRR9 appears to be primarily responsible for this light response of the circadian waves. Also, as such a light stimulus, a red light pulse that is presumably perceived by phytochromes appears to be sufficient to induce (or synchronize) the APRR1/TOC1 circadian waves.

A Complex Genetic Interaction BetweenArabidopsis thalianaTOC1 and CCA1/LHY in Driving the Circadian Clock and in Output Regulation

“…Therefore, the short-hypocotyl phenotype of the lhy cca1 double mutant under short-day conditions was dependent on TOC1 activity. toc1 coupled with cca1 or lhy is capable of light detection: It was shown before that the transcript of PRR9 rapidly and transiently accumulated when etiolated seedlings were exposed to white light and that the light-dependent acute response of PRR9 is a phytochrome-mediated event (Makino et al 2001;Ito et al 2003). Since the cca1 lhy toc1 mutant was hyposensitive to light under the photomorphogenic assay (Figure 7), it was plausible that light-induced PRR9 expression might be accordingly affected.…”

Biochemical Properties of CikA, an Unusual Phytochrome-like Histidine Protein Kinase That Resets the Circadian Clock in Synechococcus elongatus PCC 7942

“…The N-terminal domain of the circadian clock protein KaiA has been identified structurally as a pseudo-receiver that is unlikely to be involved in phosphorelay activity (51). Sequence also predicts a pseudo-receiver, unrelated to phosphoryl transfer, in the A. thaliana putative clock component TOC1 and its APRR family members (35)(36)(37)(38). We predict that each of these acts as a protein-protein interaction domain that induces conformational changes in another domain to modulate its activity.…”

“…However, the mode of action of this RR domain may be different, because the conserved Asp residue necessary for phosphoryl transfer is missing (24 -27). In this regard, the CikA structure is similar to the family of so-called pseudoresponse regulators (APRR) that has been identified in A. thaliana (35)(36)(37)(38). APRRs also possess N-terminal CONSTANS "Myb-related" motifs, which regulate transcriptional activity in eukaryotes, in addition to their pseudo-receiver domains.…”

LNK genes integrate light and clock signaling networks at the core of the Arabidopsis oscillator

“…The plant circadian clock is mostly based on clock genes that mutually regulate each other expression (6), and some of these are acutely induced by phytochromes (7)(8)(9). Interestingly, cryptochromes and phytochromes are not essential for circadian oscillations in Arabidopsis plants (10)(11)(12), but circadian regulation of phototransduction pathways generates tight links between these two signaling networks (13).…”