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The smallest known eukaryotes, at Ϸ1-m diameter, are Ostreococcus tauri and related species of marine phytoplankton. The genome of Ostreococcus lucimarinus has been completed and compared with that of O. tauri. This comparison reveals surprising differences across orthologous chromosomes in the two species from highly syntenic chromosomes in most cases to chromosomes with almost no similarity. Species divergence in these phytoplankton is occurring through multiple mechanisms acting differently on different chromosomes and likely including acquisition of new genes through horizontal gene transfer. We speculate that this latter process may be involved in altering the cell-surface characteristics of each species. In addition, the genome of O. lucimarinus provides insights into the unique metal metabolism of these organisms, which are predicted to have a large number of selenocysteine-containing proteins. Selenoenzymes are more catalytically active than similar enzymes lacking selenium, and thus the cell may require less of that protein. As reported here, selenoenzymes, novel fusion proteins, and loss of some major protein families including ones associated with chromatin are likely important adaptations for achieving a small cell size.green algae ͉ picoeukaryote ͉ genome evolution ͉ selenium ͉ synteny P hytoplankton living in the oceans perform nearly half of total global photosynthesis (1). Eukaryotic phytoplankton exhibit great diversity that contrasts with the lower apparent diversity of ecological niches available to them in aquatic ecosystems. This observation, know as the ''paradox of the plankton,'' has long puzzled biologists (2). By providing molecular level information on related species, genomics is poised to provide new insights into this paradox.Picophytoplankton, with cell diameters Ͻ2 m, play a significant role in major biogeochemical processes, primary productivity, and food webs, especially in oligotrophic waters. Within this size class, the smallest known eukaryotes are Ostreococcus tauri and related species. Although more similar to flattened spheres in shape, these organisms are Ϸ1 m in diameter (3, 4) and have been isolated or detected from samples of diverse geographical origins (5-8). They belong to the Prasinophyceae, an early diverging class within the green plant lineage, and have a strikingly simple cellular organization, with no cell wall or flagella, and with a single chloroplast and mitochondrion (4). Recent work has shown that small-subunit rDNA sequences of Ostreococcus from cultures and environmental samples cluster into four different clades that are likely distinct enough to represent different species (6, 9).Here we report on the gene content, genome organization, and deduced metabolic capacity of the complete genome of Ostreococcus sp. strain CCE9901 (7), a representative of surface-ocean adapted Ostreococcus, referred to here as Ostreococcus lucimarinus. We compare it to the analogous features of the related species O. tauri strain OTH95 (10). Our results show that many process...

Plants respond to low levels of UV-B radiation with a coordinated photomorphogenic response that allows acclimation to this environmental stress factor. The key players in this UV-B response are COP1 (an E3 ubiquitin ligase), UVR8 (a β-propeller protein), and HY5 (a bZIP transcription factor). We have shown previously that an elevated UV-B-specific response is associated with dwarf growth, indicating the importance of balancing UV-B-specific signaling. Negative regulators of this pathway are not known, however. Here, we describe two highly related WD40-repeat proteins, REPRESSOR OF UV-B PHOTOMORPHOGENESIS 1 (RUP1) and RUP2, that interact directly with UVR8 as potent repressors of UV-B signaling. Both genes were transcriptionally activated by UV-B in a COP1-, UVR8-, and HY5-dependent manner. rup1 rup2 double mutants showed an enhanced response to UV-B and elevated UV-B tolerance after acclimation. Overexpression of RUP2 resulted in reduced UV-B-induced photomorphogenesis and impaired acclimation, leading to hypersensitivity to UV-B stress. These results are consistent with an important regulatory role for RUP1 and RUP2, which act downstream of UVR8-COP1 in a negative feedback loop impinging on UVR8 function, balancing UV-B defense measures and plant growth.abiotic stress | light signaling | photobiology | quercetin | sun simulator

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Plants require the UV-B photoreceptor UV RESISTANCE LOCUS 8 (UVR8) for acclimation and survival in sunlight. Upon UV-B perception, UVR8 switches instantaneously from a homodimeric to monomeric configuration, which leads to interaction with the key signaling protein CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) and induction of UV-B-protective responses. Here, we show that UVR8 monomerization is reversible in vivo, restoring the homodimeric ground state. We also demonstrate that the UVR8-interacting proteins REPRESSOR OF UV-B PHOTOMORPHOGENESIS (RUP)1 and RUP2 mediate UVR8 redimerization independently of COP1. UVR8 redimerization consequently disrupts the UVR8-COP1 interaction, which halts signaling. Our results identify a key role of RUP1-and RUP2-mediated UVR8 redimerization in photoreceptor inactivation, a crucial process that regenerates reactivatable UVR8 homodimers.light signaling | photobiology | signal transduction U V-B radiation (UV-B; 280-315 nm) is an integral part of sunlight with a strong impact on terrestrial ecosystems (1-3). In plants, UV-B perception is necessary for UV-B acclimation and UV-B stress tolerance (4-6). Specific UV-B perception is facilitated by the photoreceptor UV RESISTANCE LOCUS 8 (UVR8) identified only recently in Arabidopsis (7). In agreement with its photoreceptor function, uvr8 null mutants show a strongly reduced response to UV-B (8-11), which even is absent under conditions specifically activating UV-B photoreceptor responses (4). In contrast, UV-B stress responses are not affected per se in uvr8 mutants (12).Upon UV-B irradiation, UVR8 homodimers monomerize instantaneously to active monomers (7). The UVR8 monomer then interacts with the WD40-repeat domain of the E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) (4), a central regulator of light-dependent plant photomorphogenesis and also of utmost importance in UV-B signaling (13,14). COP1-UVR8 interaction is an early event in the UV-B perception and signaling pathway and essential for UV-B-dependent photomorphogenesis and acclimation (4). One of the main molecular outcomes of this interaction is an increase in protein level of the bZIP transcription factor ELONGATED HYPOCOTYL 5 (HY5), which may be the result of reduced HY5 ubiquitination by COP1 (4). HY5 together with its homolog HYH induce expression of the majority but not all genes included in the UVR8-dependent UV-B response (15-18). In a negative feedback loop, the light-regulated SALT TOLERANCE/B-BOX DOMAIN PROTEIN 24 (STO/BBX24) was shown to fine-tune the UV-B response by impinging on HY5 (19).UVR8 is a seven-bladed β-propeller protein that makes use of tryptophan residues intrinsic to the protein as chromophores for UV-B absorption, with a primary role established for tryptophan-285 (7,20,21). In agreement with the major role that Trp-285 plays in UV-B-mediated monomerization of UVR8 (7), it was proposed that UV-B absorption by specific tryptophans, namely Trp-285 and Trp-233, leads to disruption of cross-dimer salt bridges involving crucial arginins (20,21...

Ultraviolet-B radiation (UV-B)is an intrinsic part of sunlight that is accompanied by significant biological effects. Plants are able to perceive UV-B using the UV-B photoreceptor UVR8 which is linked to a specific molecular signaling pathway and leads to UV-B acclimation. Herein we review the biological process in plants from initial UV-B perception and signal transduction through to the known UV-B responses that promote survival in sunlight. The UVR8 UV-B photoreceptor exists as a homodimer that instantly monomerises upon UV-B absorption via specific intrinsic tryptophans which act as UV-B chromophores. The UVR8 monomer interacts with COP1, an E3 ubiquitin ligase, initiating a molecular signaling pathway that leads to gene expression changes. This signaling output leads to UVR8-dependent responses including UV-B-induced photomorphogenesis and the accumulation of UV-B-absorbing flavonols. Negative feedback regulation of the pathway is provided by the WD40-repeat proteins RUP1 and RUP2, which facilitate UVR8 redimerization, disrupting the UVR8-COP1 interaction. Despite rapid advancements in the field of recent years, further components of UVR8 UV-B signaling are constantly emerging, and the precise interplay of these and the established players UVR8, COP1, RUP1, RUP2 and HY5 needs to be defined. UVR8 UV-B signaling represents our further understanding of how plants are able to sense their light environment and adjust their growth accordingly.

Members of the cryptochrome/photolyase family (CPF) are widely distributed throughout all kingdoms, and encode photosensitive proteins that typically show either photoreceptor or DNA repair activity. Animal and plant cryptochromes have lost DNA repair activity and now perform specialized photoperceptory functions, for example, plant cryptochromes regulate growth and circadian rhythms, whereas mammalian and insect cryptochromes act as transcriptional repressors that control the circadian clock. However, the functional differentiation between photolyases and cryptochromes is now being questioned. Here, we show that the PtCPF1 protein from the marine diatom Phaeodactylum tricornutum shows 6-4 photoproduct repair activity and can act as a transcriptional repressor of the circadian clock in a heterologous mammalian cell system. Conversely, it seems to have a wide role in blue-light-regulated gene expression in diatoms. The protein might therefore represent a missing link in the evolution of CPFs, and act as a novel ultraviolet/blue light sensor in marine environments.