Molecular Plant Advance Access published online on October 11, 2007
Molecular Plant, doi:10.1093/mp/ssm005
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© The Author 2007. Published by Oxford University Press.
The Clock Protein CCA1 and the bZIP Transcription Factor HY5 Physically Interact to Regulate Gene Expression in Arabidopsis
Department of Molecular, Cell and Developmental Biology, University of California, Los Angeles, CA 90095–1606, USA
1 To whom correspondence should be addressed. E-mail etobin{at}ucla.edu, tel. 1–310–825–7700, fax 1–310–206–3987.
The circadian clock regulates the expression of an array of Arabidopsis genes such as those encoding the LIGHT-HARVESTING CHLOROPHYLL A/B (Lhcb) proteins. We have previously studied the promoters of two of these Arabidopsis genes—Lhcb1*1 and Lhcb1*3—and identified a sequence that binds the clock protein CIRCADIAN CLOCK ASSOCIATED 1 (CCA1). This sequence, designated CCA1-binding site (CBS), is necessary for phytochrome and circadian responsiveness of these genes. In close proximity to this sequence, there exists a G-box core element that has been shown to bind the bZIP transcription factor HY5 in other light-regulated plant promoters. In the present study, we examined the importance of the interaction of transcription factors binding the CBS and the G-box core element in the control of normal circadian rhythmic expression of Lhcb genes. Our results show that HY5 is able to specifically bind the G-box element in the Lhcb promoters and that CCA1 can alter the binding activity of HY5. We further show that CCA1 and HY5 can physically interact and that they can act synergistically on transcription in a yeast reporter gene assay. An absence of HY5 leads to a shorter period of Lhcb1*1 circadian expression but does not affect the circadian expression of CATALASE3 (CAT3), whose promoter lacks a G-box element. Our results suggest that interaction of the HY5 and CCA1 proteins on Lhcb promoters is necessary for normal circadian expression of the Lhcb genes.
a These authors contributed equally to this work.
b Present address: Biovista, 34 Rodopoleos St, 16777, Athens, Greece.
c Present address: Albert Katz Dept. Drylands Biotechnology, Jacob Blaustein Insts. for Desert Research, Ben–Gurion University of the Negev, Midreshet Ben–Gurion, 84990, Israel.
d Present address: Plant Disease Resistance Research Unit, Division of Plant Sciences, National Institute of Agrobiological Sciences, Tsukuba, Ibaraki 305-8602, Japan.
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