Classical scholars interested by the rhythmic movements of some plants' leaves made the first records of daily biological rhythms. The mechanism underlying their 24-hour timing is now known as the circadian system or circadian clock. Recent studies have uncovered three members of a small protein family that control development and may be novel components of the clock in the model plant Arabidopsis thaliana ([1,2] and M. Wada, personal communication in The circadian clocks of plants and almost all other eukaryotes behave very similarly: all have a rhythmic period close to 24 hours and can be reset by changes in ambient light. Studies of mutant insects, fungi, cyanobacteria and rodents with altered or absent timing have identified several genes involved in negative feedback loops, through which a few proteins rhythmically regulate the transcription of their cognate genes Routes around the clock Clock genetics in higher plants was uneventful until the 1990s. Then, staying up late in the lab, Nagy and Kay in Chua's group serendipitously found that the circadian clock controlled transcription from the chlorophyll a/b-binding protein (CAB) promoter, which was a workhorse of plant molecular biology. Kay proposed to find clock mutant plants, based upon their mis-timed CAB transcription. The other proposed ingredients were a bioluminescent firefly luciferase (luc) reporter gene, which might allow CAB:luc transgenic plants to glow rhythmically, and a photon-counting camera (initially located in Singapore). We identified our first circadian clock mutants in the latter half of 1992, by screening mutagenised CAB:luc plants for mis-timed luminescence Clock mutants of Arabidopsis are now arriving like buses. The circadian clock is a genome-wide regulator that regulates many processes, such as flowering and the growth of the seedling stem or hypocotyl. Genetic screens for mutants that mis-regulate any of these processes may therefore recover clock-associated genes. Targeted screens are not even required, because long-hypocotyl seedlings and plants with altered flowering time -like the fkf mutant [2] -stand out in any genetic screen in Arabidopsis. Biochemical or molecular screening for proteins that regulate gene expression can likewise uncover clock proteins, if the clock controls the gene of interest. The 'circadian clock associated' (CCA1) protein was identified by its binding to the CAB promoter, but itself turns out to be rhythmically expressed and involved in a clock-like negative feedback loop As the Arabidopsis genome sequence rolled out in 1999, the third member of the ZTL family, LKP2, was identified by its homology of part of its protein product to the LOV domain (M. Wada, personal communication in Sequence relationships The LOV sequence near the amino terminus of ZT
