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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

Identification of Negative cis-Acting Elements in Response to Copper in the Chloroplastic Iron Superoxide Dismutase Gene of the Moss Barbula unguiculata¹

Department of Biological Science, Graduate School of Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima 739–8526, Japan

Superoxide dismutases (SODs) are ubiquitous metalloenzymes that catalyze the dismutation of superoxide radicals. Chloroplasts have two isozymes, copper/zinc SOD (Cu/ZnSOD) and iron SOD (FeSOD), encoded by nuclear genes. Because bryophytes are considered as the earliest land plants, they are one of the most interesting plant models for adaptation against oxidative stress. In a previous study, we found that the FeSOD gene was expressed under Cu-deficient conditions and repressed under high-Cu-supply conditions; on the other hand, the Cu/ZnSOD gene was induced by Cu in a moss, Barbula unguiculata. The expression of Cu/ZnSOD and FeSOD is coordinately regulated at the transcriptional level depending on metal bioavailability. Here, using transgenic moss plants, we determined that the GTACT motif is a negative cis-acting element of the moss FeSOD gene in response to Cu. Furthermore, we found that a plant-specific transcription factor, PpSBP2 (for SQUAMOSA promoter-binding protein), and its related proteins bound to the GTACT motif repressed the expression of the FeSOD gene. The moss FeSOD gene was negatively regulated by Cu in transgenic Nicotiana tabacum plants, and the Arabidopsis thaliana FeSOD gene promoter containing the GTACT motif was repressed by Cu. Our results suggested that molecular mechanisms of GTACT motif-dependent transcriptional suppression by Cu are conserved in land plants.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instruction for Authors is: Yohsuke Takahashi (ytakahas{at}hiroshima-u.ac.jp).

www.plantphysiol.org/cgi/doi/10.1104/pp.107.114868

^* Corresponding author; e-mail ytakahas{at}hiroshima-u.ac.jp.

Received December 11, 2007; accepted February 2, 2008; published February 7, 2008.