SPA1, a WD-Repeat Protein Specific to Phytochrome A Signal Transduction

The five members of the phytochrome photoreceptor family of Arabidopsis thaliana control morphogenesis differentially in response to light. Genetic analysis has identified a signaling pathway that is specifically activated by phytochrome A. A component in this pathway, SPA1 (for "suppressor of phyA-105"), functions in repression of photomorphogenesis and is required for normal photosensory specificity of phytochrome A. Molecular cloning of the SPA1 gene indicates that SPA1 is a WD (tryptophan-aspartic acid)-repeat protein that also shares sequence similarity with protein kinases. SPA1 can localize to the nucleus, suggesting a possible function in phytochrome A-specific regulation of gene expression.