The serine-rich N-terminal region of Arabidopsis phytochrome A is required for protein stability
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Deletion or substitution of the serine-rich N-terminal stretch of grass phytochrome A (phyA) has repeatedly been shown to yield a hyperactive photoreceptor when expressed under the control of a constitutive promoter in transgenic tobacco or Arabidopsis seedlings retaining their native phyA. These observations have lead to the proposal that the serine-rich region is involved in negative regulation of phyA signaling. To re-evaluate this conclusion in a more physiological context we produced transgenic Arabidopsis seedlings of the phyA-null background expressing Arabidopsis PHYA deleted in the sequence corresponding to amino acids 6–12, under the control of the native PHYA promoter. Compared to the transgenic seedlings expressing wild-type phyA, the seedlings bearing the mutated phyA showed normal responses to pulses of far-red (FR) light and impaired responses to continuous FR light. In yeast two-hybrid experiments, deleted phyA interacted normally with FHY1 and FHL, which are required for phyA accumulation in the nucleus. Immunoblot analysis showed reduced stability of deleted phyA under continuous red or FR light. The reduced physiological activity can therefore be accounted for by the enhanced destruction of the mutated phyA. These findings do not support the involvement of the serine-rich region in negative regulation but they are consistent with a recent report suggesting that phyA turnover is regulated by phosphorylation.
KeywordsHigh-irradiance response Light signaling Phytochrome A Protein degradation Serine-rich domain
Red-light-absorbing form of phytochrome
FR-absorbing form of phytochrome
We thank Eberhard Schäfer (University of Freiburg) for allowing us to do the yeast two-hybrid experiments in his lab. This work was supported by the National Agency for the promotion of Science and Technology of Argentina (ANPCYT, grant BID 1728/OC-AR PICT 11631 to JJC), by University of Buenos Aires (grant G021 to JJC), by the Swiss National Science Foundation (grant PP00A−103005 to CF), the Human Frontier Science Program (HFSP) (grant RGY0016/2004-C to CF) and the University of Lausanne (to CF); by a fellowship from the HFSP to AH (LT00631/2003-C).
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