Light Controls Protein Localization through Phytochrome-Mediated Alternative Promoter Selection

Alternative promoter usage is a proteome-expanding mechanism that allows multiple pre-mRNAs to be transcribed from a single gene. The impact of this mechanism on the proteome and whether it is positively exploited in normal organismal responses remain unclear. Wefound that the plant photoreceptor phytochrome induces genome-wide changes in alternative promoter selection in Arabidopsis thaliana. Through this mechanism, protein isoforms with different N termini are produced that display lightdependent differences in localization. For instance, shade-grown plants accumulate a cytoplasmic isoform of glycerate kinase (GLYK), an essential photorespiration enzyme that was previously thought to localize exclusively to the chloroplast. Cytoplasmic GLYK constitutes a photorespiratory bypass that alleviates fluctuating light-induced photoinhibition. Therefore, phytochrome controls alternative promoter selection to modulate protein localization in response to changing light conditions. This study suggests that alternative promoter usage represents anotherubiquitous layer of gene expression regulation in eukaryotes that contributes to diversification of the proteome.