The interactive regulation between clock genes is central for oscillator function. Here, we show interactions between the Arabidopsis clock genes LATE ELONGATED HYPOCOTYL ( LHY ), CIRCADIAN CLOCK ASSOCIATED 1 ( CCA1 ), and TIMING OF CAB EXPRESSION 1 ( TOC1 ). The MYB transcription factors LHY and CCA1 negatively regulate TOC1 expression. We show that both proteins bind to a region in the TOC1 promoter that is critical for its clock regulation. Conversely, TOC1 appears to participate in the positive regulation of LHY and CCA1 expression. Our results indicate that these interactions form a loop critical for clock function in Arabidopsis .

The toc1 mutation causes shortened circadian rhythms in light-grown Arabidopsis plants. Here, we report the same toc1 effect in the absence of light input to the clock. We also show that TOC1 controls photoperiodic flowering response through clock function. The TOC1 gene was isolated and found to encode a nuclear protein containing an atypical response regulator receiver domain and two motifs that suggest a role in transcriptional regulation: a basic motif conserved within the CONSTANS family of transcription factors and an acidic domain. TOC1 is itself circadianly regulated and participates in a feedback loop to control its own expression.

Most organisms have circadian clocks consisting of negative feedback loops of gene regulation that facilitate adaptation to cycles of light and darkness. In this study, CRYPTOCHROME (CRY), a protein involved in circadian photoperception in Drosophila , is shown to block the function of PERIOD/TIMELESS (PER/TIM) heterodimeric complexes in a light-dependent fashion. TIM degradation does not occur under these conditions; thus, TIM degradation is uncoupled from abrogation of its function by light. CRY and TIM are part of the same complex and directly interact in yeast in a light-dependent fashion. PER/TIM and CRY influence the subcellular distribution of these protein complexes, which reside primarily in the nucleus after the perception of a light signal. Thus, CRY acts as a circadian photoreceptor by directly interacting with core components of the circadian clock.

Tic TOC1 Plant Clock: The molecular clocks intertwined with cellular physiology regulate daily cycles. In plants, these circadian rhythms affect processes as diverse as carbon metabolism and leaf orientation. Huang et al. (p. 75 , published online 8 March) have now analyzed the interactions driven by a key element of the circadian clock in plants, TOC1 (“TIMING OF CAB EXPRESSION 1”). TOC1 helps to coordinate responses of the morning and evening cycles, functioning to repress activity of other clock components.