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J. Biol. Chem., Vol. 280, Issue 49, 40820-40831, December 9, 2005

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Isolation of a Calmodulin-binding Transcription Factor from Rice (Oryza sativa L.)^*

Man Soo Choi12, Min Chul Kim¶1, Jae Hyuk Yoo, Byeong Cheol Moon, Sung Cheol Koo2, Byung Ouk Park, Ju Huck Lee, Yoon Duck Koo, Hay Ju Han2, Sang Yeol Lee, Woo Sik Chung, Chae Oh Lim3, and Moo Je Cho4

From the Division of Applied Life Science (BK21 Program), Plant Molecular Biology and Biotechnology Research Center and Environmental Biotechnology National Core Research Center, ^¶Division of Plant Resources and Environment, Gyeongsang National University, Jinju 660-701, Korea

Calmodulin (CaM) regulates diverse cellular functions by modulating the activities of a variety of enzymes and proteins. However, direct modulation of transcription factors by CaM has been poorly understood. In this study, we isolated a putative transcription factor by screening a rice cDNA expression library by using CaM:horse-radish peroxidase as a probe. This factor, which we have designated OsCBT (Oryza sativa CaM-binding transcription factor), has structural features similar to Arabidopsis AtSRs/AtCAMTAs and encodes a 103-kDa protein because it contains a CG-1 homology DNA-binding domain, three ankyrin repeats, a putative transcriptional activation domain, and five putative CaM-binding motifs. By using a gel overlay assay, gel mobility shift assays, and site-directed mutagenesis, we showed that OsCBT has two different types of functional CaM-binding domains, an IQ motif, and a Ca²⁺-dependent motif. To determine the DNA binding specificity of OsCBT, we employed a random binding site selection method. This analysis showed that OsCBT preferentially binds to the sequence 5'-TWCG(C/T)GTKKKKTKCG-3' (W and K represent A or C and T or G, respectively). OsCBT was able to bind this sequence and activate -glucuronidase reporter gene expression driven by a minimal promoter containing tandem repeats of these sequences in Arabidopsis leaf protoplasts. Green fluorescent protein fusions of two putative nuclear localization signals of OsCBT, a bipartite and a SV40 type, were predominantly localized in the nucleus. Most interestingly, the transcriptional activation mediated by OsCBT was inhibited by co-transfection with a CaM gene. Taken together, our results suggest that OsCBT is a transcription activator modulated by CaM.

Received for publication, April 27, 2005 , and in revised form, September 16, 2005.

^* This work was supported in part by Environmental Biotechnology National Core Research Center Grant R15-2003-012-01001-0 funded by KOSEF/MOST, Plant Diversity Research Center Grant PF0330402-00 of 21st Century Frontier Research Program funded by MOST, and in part by Gyeongnam Biotechnology Program funded by Gyeongsangnamdo and BioGreen21 program funded by Rural Development Administration. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ Both authors contributed equally to this work.

² Supported by a scholarship from the BK21 Program of the Ministry of Education and Human Resource Development.

³ To whom correspondence may be addressed. Tel.: 82-55-751-6255; Fax: 82-55-759-9363; E-mail: colim{at}gsnu.ac.kr.

⁴ To whom correspondence may be addressed. Tel.: 82-55-751-6146; Fax: 82-55-759-9363; E-mail: choslab{at}gsnu.ac.kr.

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