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.

The cycling bioluminescence of Arabidopsis plants carrying a firefly luciferase fusion construct was used to identify mutant individuals with aberrant cycling patterns. Both long- and short-period mutants were recovered. A semidominant short-period mutation, timing of CAB expression (toc1), was mapped to chromosome 5. The toc1 mutation shortens the period of two distinct circadian rhythms, the expression of chlorophyll a/b-binding protein (CAB) genes and the movements of primary leaves, although toc1 mutants do not show extensive pleiotropy for other phenotypes.

We have used a luciferase reporter gene and continuous automated monitoring of bioluminescence to demonstrate unequivocally that cyanobacteria exhibit circadian behaviors that are fundamentally the same as circadian rhythms of eukaryotes. We also show that these rhythms can be studied by molecular methods in Synechococcus sp. PCC7942, a strain for which genetic transformation is well established. A promoterless segment of the Vibrio harveyi luciferase structural genes (luxAB) was introduced downstream of the promoter for the Synechococcus psbAI gene, which encodes a photosystem H protein. This reporter construction was recombined into the Synechococcus chromosome, and bioluminescence was monitored under conditions of constant illumination following entrainment to light and dark cycles. The reporter strain, AMC149, expressed a rhythm of bioluminescence which satisfies the criteria of circadian rhythms: persistence in constant conditions, phase resetting by light/dark signals, and temperature compensation of the period. Rhythmic changes in levels of the native psbAl message following light/dark entrainment supported the reporter data.The behavior of this prokaryote disproves the dogma that circadian mechanisms must be based on eukaryotic cellular organization. Moreover, the cyanobacterial strain described here provides an efficient experimental system for molecular analysis of the circadian clock.Despite decades of study, the biochemical mechanism of circadian clocks remains a mystery. Circadian rhythms have been found in a wide spectrum of organisms (1) but, until recently, only in eukaryotes (2, 3). In the last few years circadian rhythms have been reported in the prokaryotic cyanobacteria (4-7). Unfortunately, these studies employed genetically intractable cyanobacterial strains and laborious assays to detect the rhythms. These difficulties have impeded the demonstration that the prokaryotic rhythms are equivalent to the circadian rhythms of eukaryotes.Proof that prokaryotes have circadian pacemakers has threefold significance. (i) With regard to the evolutionary emergence of circadian behavior: Can the simpler organization of prokaryotes support a circadian mechanism? Is circadian behavior adaptive for prokaryotic niches as well as for eukaryotic niches? (ii) The previous failure of attempts to discover circadian clocks in prokaryotes has led to a "eukaryotes-only" dogma which limited the types ofmodels that have been considered for the underlying clock mechanism (3). Now that prokaryotic cellular organization appears to be fully competent to generate circadian oscillations, a broader range of mechanisms can be seriously evaluated as candidates for the circadian pacemaker. (iii) The realization thatThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.prokaryotes express circadian behavior is significant from the perspective of designing an optimal s...