The Plant Cell, Vol. 9, 491-507, April 1997 O 1997 American Society of Plant Physiologists A Myb-Related Transcription Factor zyx 1s lnvolved in the Phytochrome Regulation of an Arabidopsis zyx Lhcb Gene Zhi-Yong Wang, David Kenigsbuch,' Lin Sun,2 Eitan Harel,3 May S. Ong, and Elaine M. Tobin4 Department of Molecular, Cellular, and Developmental Biology, University of California, Los Angeles, California 90095-1 606 We have isolated the gene for a protein designated CCAl. This protein can bind to a region of the promoter of an Arabi- dopsis light-harvesting chlorophyll a/b protein gene, Lhcb7*3, which is necessary for its regulation by phytochrome. The CCAl protein interacted with two imperfect repeats in the Lhcb7*3 promoter, AAA/,AATCT, a sequence that is con- served in Lhcb genes. A region near the N terminus of CCA1, which has some homology to the repeated sequence found in the DNA binding domain of Myb proteins, is required for binding to the Lhcb7*3 promoter. Lines of transgenic Arabi- dopsis plants expressing antisense RNA for CCAl showed reduced phytochrome induction of the endogenous Lhcb7*3 gene, whereas expression of another phytochrome-regulated gene, rbcS-7A, which encodes the small subunit of ribu- lose-1,Bbisphosphate carboxylase/oxygenase, was not affected. Thus, the CCAl protein acts as a specific activator of Lhcb7*3 transcription in response to brief red illumination. The expression of CCAl RNA was itself transiently increased when etiolated seedlings were transferred to light. We conclude that the CCAl protein is a key element in the function- ing of the phytochrome signal transduction pathway leading to increased transcription of this Lhcb gene in Arabidopsis. INTRODUCTION Plants use severa1 different photoreceptors to respond to changes in environmental conditions. Among the best stud- ied of these photoreceptors are the phytochromes, compris- ing a family of closely related chromoproteins that are involved in regulating a diverse array of developmental pro- cesses in higher plants, including seed germination, photo- morphogenesis, and flowering (Kendrick and Kronenberg, 1994). Phytochromes can exist in either of two photochemi- cally interconvertible forms: Pr (absorption maximum in the red region of the spectrum) and Pfr (absorption maximum in the far-red region of the spectrum). A classical diagnostic hallmark of a phytochrome response is that it can be elicited by brief red illumination (R), and the effect of R can be sub- stantially reversed by far-red illumination (FR) given immedi- ately after an R treatment. Although the primary mechanism of action of any phytochrome is not known, activation of phytochrome signal transduction by light has been shown to affect transcription of a number of different genes. Different phytochrome family members can have redundant func- tions, and at least three phytochrome types can regulate the expression of genes encoding light-harvesting chlorophyll a/b Current address: Life Sciences Department, Bar llan University, Ramat Gan, Israel. * Current address: Department of Molecular Biology, Massachusetts General Hospital, Boston, MA 021 14. Current address: Department of Plant Sciences, Hebrew Univer- sity, Jerusalem 91904, Israel. 4To whom correspondence should be addressed. E-mail etobin @ucla.edu; fax 31 0-206-4386. proteins associated with photosystem I1 (Lhcb genes) (Furuya and Schafer, 1996). A range of experimental approaches has been taken to understand the ways in which phytochrome acts and to identify components involved in transducing the original light signal. There is biochemical and pharmacological evi- dente that G proteins and a Caz+/calmodulin system are in- volved in phytochrome signal transduction mechanisms (reviewed in Deng, 1994; Barnes et al., 1995; Quail et al., 1995). Proteins affecting processes that can be regulated by phytochrome have been identified by the mutation of genes (designated zyxwvu DETlOLATED [DET]/CONSTITUT/V€ PHOTO- MORPHOGENlC [COP]/fUSCA [FUS]) that have pleiotropic effects, including repression of photomorphogenesis. Sev- era1 of these genes have now been cloned and character- ized (reviewed in Chory et al., 1996; von Arnim and Deng, 1996). The proteins identified by this strategy include those that are involved in a diverse array of responses and not necessarily directly involved in mediating the action of phy- tochrome, but there is evidence that the DET7 and COP7 genes may play a specific role as repressors in photomor- phogenesis (Chory et al., 1996; Wei and Deng, 1996). Another approach to understanding the phytochrome sig- na1 transduction chain isto identify promoter sequences and the cognate factors mediating the regulation of transcription. There have been many studies of promoter regions respon- sible for differences in gene expression between light-grown and dark-treated plants, but fewer studies have focused specifically on phytochrome regulation (Tobin and Kehoe, 1994; Terzaghi and Cashmore, 1995).Studies that have defined