Regulation of copper homeostasis by micro-RNA in Arabidopsis.

Major copper proteins in the cytoplasm of plant cells are plastocyanin, copper/zinc superoxide dismutase, and cytochrome c oxidase. Under copper limited conditions, expression of copper/zinc superoxide dismutase is down-regulated and the protein is replaced by iron superoxide dismutase in chloroplasts. We present evidence that a micro-RNA, miR398, mediates this regulation in Arabidopsis thaliana, by directing the degradation of copper/zinc superoxide dismutase mRNA when copper is limited. Sequence analysis indicated that the transcripts encoding cytosolic copper/zinc superoxide dismutase and COX5b-1, a subunit of the mitochondrial cytochrome c oxidase, are also targeted by miR398. This regulation via miR398 takes place in response to changes in a low range of copper levels (0.2-0.5 microM), indicating that miR398 is involved in a response to copper limitation. On the other hand, another major copper protein, plastocyanin, which is involved in photosynthetic electron flow and is essential in higher plants, was not regulated via miR398. We propose that miR398 is a key factor in copper homeostasis in plants and regulates the stability of mRNAs of major copper proteins under copper-limited conditions.