<i>PHYTOCLOCK 1</i> encoding a novel GARP protein essential for the <i>Arabidopsis</i> circadian clock

<jats:p>Previously, we screened 50 000 seedlings of <jats:italic>Arabidopsis thaliana</jats:italic> carrying a P<jats:italic><jats:sub>GI</jats:sub></jats:italic>::<jats:italic>LUC</jats:italic><jats:sup>+</jats:sup> bioluminescence reporter gene mutagenized with ethylmethanesulfonate for mutants with phenotypes of extensively altered circadian rhythms, and identified three loci, <jats:italic>PHYTOCLOCK 1</jats:italic> (<jats:italic>PCL1</jats:italic>), <jats:italic>PCL2</jats:italic> and <jats:italic>PCL3</jats:italic>, whose mutations cause arrhythmia. Here we succeeded to clone the <jats:italic>PCL1</jats:italic> gene and show that the <jats:italic>PCL1</jats:italic> gene encodes a novel DNA binding protein belonging to the GARP protein family and is essential for a functional clock oscillator in <jats:italic>A. thaliana</jats:italic>. The <jats:italic>PCL1</jats:italic> gene satisfies the requirements for the clock oscillator gene: (i) <jats:italic>pcl1</jats:italic> null mutations caused arrhythmia in multiple circadian outputs, including expression of potential clock genes <jats:italic>TOC1</jats:italic>, <jats:italic>CCA1</jats:italic> and <jats:italic>LHY</jats:italic>, and flowering lacked a photoperiodic response; (ii) <jats:italic>PCL1</jats:italic> expression showed circadian rhythm in both constant light and constant dark; (iii) over‐expression of the <jats:italic>PCL1</jats:italic> gene gradually caused arrhythmicity in all the multiple circadian outputs examined; and (iv) the <jats:italic>PCL1</jats:italic> gene controlled its own expression <jats:italic>via</jats:italic> negative feedback. Therefore, the <jats:italic>PCL1</jats:italic> gene is the clock oscillator gene essential to the generation of clock oscillation in the higher plant.</jats:p>