Shedding (blue) light on algal gene expression.

L ight in the blue region of the spectrum [blue light (BL), 400– 480 nm] is an ubiquitous environmental signal. BL can penetrate marine water to a depth greater than all other wavelengths, up to the limits of the photic zone ( 1,500 m depth) and may be linked to the evolution of photosynthesis (1). BL is also potentially dangerous because it is readily absorbed by intracellular photosensitizers (e.g., porphyrin derivatives and flavins) (2). Therefore, living organisms detect and respond to BL either by photoprotection mechanisms or by maximally exploiting this environmental input, e.g., to entrain circadian rhythms and optimize photosynthetic efficiency. Given its high penetrability, BL is of utmost importance for marine species, but little is known about the BL detection mechanisms of sea plants. The AUREOCHROMES (AUREOs) described by Takahashi and colleagues (3) in a recent issue of PNAS are the first BL receptors identified in stramenopile algae and show a clear link to the photomorphogenesis of these organisms. The existence of BL photoreceptors in plants has long been proposed, but only recently have the flavin-binding cryptochromes and phototropins (phot) been characterized at a molecular level (4). Phot are conserved in higher plants and in several lower plant species, where they mediate a variety of BL responses (e.g., phototropism, gametogenesis) (5). What is making this research field increasingly exciting is the awareness that BL photoreceptors are widespread among distant taxa and are well represented in both eukaryotes and prokaryotes (4, 6, 7). The common feature conserved among phot-related proteins is the light-sensing, f lavin-binding light– oxygen–voltage (LOV) domain, a small protein unit of 110 aa belonging to the PerArntSim (PAS) superfamily (8). Takahashi and colleagues (3) now show the presence of LOV proteins in the photosynthetic stramenopiles Vaucheria frigida and Fucus distichus and in a diatomean species, Thalassiosira pseudonana. These novel phot-related proteins have been named AUREO in reference to the typical golden-yellow color of stramenopiles. The authors identified the sequence of AUREOs, their photoinduced reactions, and their light-driven regulation of gene expression and photomorphogenesis in V. frigida. This represents an important step in the understanding of photoreceptor systems and BL-driven responses in these marine plants, which originated by means of secondary endosymbiosis from red algal symbionts and nonphotosynthetic eukaryotic hosts. The finding of AUREOs could provide new information on the phylogenetic link between these plants and other eukaryotes. V. frigida hosts two AUREOs (AUREO1 and AUREO2) composed of an N-terminal, DNA-binding basic leucine zipper (bZIP) motif and a Cterminal LOV domain (Fig. 1a), but it does not possess phot-encoding genes. Conversely, a search through various plant genomes rules out AUREO-like proteins in green plants. AUREO1 shows the spectral features and lightinduced reactions typical of the well known LOV paradigm. LOV domains noncovalently bind a fully oxidized flavin mononucleotide (FMN) molecule in the dark, absorbing maximally at 447 nm. BL triggers a photocycle that involves the transient formation of an FMN–cysteine C(4a) thiol adduct, slowly reverting to the dark state on a seconds-to-hours timescale (7). The architecture of AUREO underscores the modularity of LOV proteins. In phot, two LOV domains (LOV1 and LOV2) are coupled to a kinase effector module whose activity is enhanced upon light activation. LOV2 is necessary and sufficient for such light activation, which leads us to question the role of LOV1. In all other LOV proteins, only one LOV domain is present, coupled to a broad variety of effector functions, such as kinases and transcriptional regulators, constituting modular systems presumably switchable by light. Other than phot, fungal BL sensors of the LOV family are the best understood systems (9), but recently, bacterial LOV proteins have also begun to be examined. YtvA from Bacillus subtilis is the first bacterial protein for which the LOV paradigm has been demonstrated, followed by LOV proteins from proteobacteria and cyanobacteria (7, 10). At the functional level, light excitation increases the level of phosphorylation in bacterial LOV kinases, showing that a typical bacterial two-component system can be BLactivated (10–12). In Brucella abortus, this light-regulated kinase activity was importantly linked to the infectivity of the bacterium for mammalian cells (12). In Caulobacter crescentus, a LOV kinase is the BL receptor that regulates cell– cell attachment (10). Takahashi and