Illumination changes elicit modifications of thylakoid proteins and reorganization of the photosynthetic machinery. This involves, in the short term, phosphorylation of photosystem II (PSII) and light-harvesting (LHCII) proteins. PSII phosphorylation is thought to be relevant for PSII turnover 1,2 , whereas LHCII phosphorylation is associated with the relocation of LHCII and the redistribution of excitation energy (state transitions) between photosystems 3,4 . In the long term, imbalances in energy distribution between photosystems are counteracted by adjusting photosystem stoichiometry 5,6 . In the green alga Chlamydomonas and the plant Arabidopsis, state transitions require the orthologous protein kinases STT7 and STN7, respectively 7,8 . Here we show that in Arabidopsis a second protein kinase, STN8, is required for the quantitative phosphorylation of PSII core proteins. However, PSII activity under high-intensity light is affected only slightly in stn8 mutants, and D1 turnover is indistinguishable from the wild type, implying that reversible protein phosphorylation is not essential for PSII repair. Acclimation to changes in light quality is defective in stn7 but not in stn8 mutants, indicating that short-term and long-term photosynthetic adaptations are coupled. Therefore the phosphorylation of LHCII, or of an unknown substrate of STN7, is also crucial for the control of photosynthetic gene expression.STT7 and STN7 are orthologous protein kinases required for LHCII phosphorylation and for state transitions in Chlamydomonas and Arabidopsis, respectively 7,8 . In Arabidopsis, another STT7/STN7-like protein (STN8) exists that is not required for state transitions 8 . STN8 is located in the chloroplast, as shown by in vivo subcellular localization of its amino-terminal region fused to the dsRED protein and by the import of, and transit peptide removal from, STN8 translated in vitro (Fig. 1a, b). Chloroplast subfractionation after import revealed that the protein is associated, like STT7 and STN7, with thylakoids ( Fig. 1c) (refs 7, 8).Insertion mutants for STN8 and STN7 were obtained from the Salk collection 9 , and for each gene two independent mutant alleles lacking the respective transcript were identified (Supplementary Fig. S1). The stn7 stn8 double mutant was generated by crossing stn7 and stn8 single knockouts and screening the resulting F 2 generation for homozygous double mutants. All mutants were indistinguishable from the wild type with regard to the timing of seed germination and growth rate in the greenhouse ( Supplementary Fig. S1). In stn7 and stn7 stn8 mutants, a slight decrease in the levels of neoxanthin, lutein and total chlorophyll was found (Supplementary Table S1). These subtle changes can be attributed to a minor decrease in LHCII content, not detectable by polyacrylamide-gel electrophoresis (PAGE) analysis ( Supplementary Fig. S2).Photosynthetic electron flow, measured on the basis of chlorophyll fluorescence, was not altered in the mutants (Supplementary Table S2). State transitions w...

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Flowering plants control energy allocation to their photosystems in response to light quality changes. This includes the phosphorylation and migration of light-harvesting complex II (LHCII) proteins (state transitions or short-term response) as well as long-term alterations in thylakoid composition (long-term response or LTR). Both responses require the thylakoid protein kinase STN7. Here, we show that the signaling pathways triggering state transitions and LTR diverge at, or immediately downstream from, STN7. Both responses require STN7 activity that can be regulated according to the plastoquinone pool redox state. However, LTR signaling does not involve LHCII phosphorylation or any other state transition step. State transitions appear to play a prominent role in flowering plants, and the ability to perform state transitions becomes critical for photosynthesis in Arabidopsis thaliana mutants that are impaired in thylakoid electron transport but retain a functional LTR. Our data imply that STN7-dependent phosphorylation of an as yet unknown thylakoid protein triggers LTR signaling events, whereby an involvement of the TSP9 protein in the signaling pathway could be excluded. The LTR signaling events then ultimately regulate in chloroplasts the expression of photosynthesis-related genes on the transcript level, whereas expression of nuclear-encoded proteins is regulated at multiple levels, as indicated by transcript and protein profiling in LTR mutants.

The major RNA polymerase activity in mature chloroplasts is a multisubunit, Escherichia coli-like protein complex called PEP (for plastid-encoded RNA polymerase). Its subunit structure has been extensively investigated by biochemical means. Beside the "prokaryotic" subunits encoded by the plastome-located RNA polymerase genes, a number of additional nucleus-encoded subunits of eukaryotic origin have been identified in the PEP complex. These subunits appear to provide additional functions and regulation modes necessary to adapt transcription to the varying functional situations in chloroplasts. However, despite the enormous progress in genomic data and mass spectrometry techniques, it is still under debate which of these subunits belong to the core complex of PEP and which ones represent rather transient or peripheral components. Here, we present a catalog of true PEP subunits that is based on comparative analyses from biochemical purifications, protein mass spectrometry, and phenotypic analyses. We regard reproducibly identified protein subunits of the basic PEP complex as essential when the corresponding knockout mutants reveal an albino or pale-green phenotype. Our study provides a clearly defined subunit catalog of the basic PEP complex, generating the basis for a better understanding of chloroplast transcription regulation. In addition, the data support a model that links PEP complex assembly and chloroplast buildup during early seedling development in vascular plants.

Excitation imbalances between photosystem I and II generate redox signals in the thylakoid membrane of higher plants which induce acclimatory changes in the structure of the photosynthetic apparatus. They affect the accumulation of reaction center and light-harvesting proteins as well as chlorophylls a and b. In Arabidopsis thaliana the re-adjustment of photosystem stoichiometry is mainly mediated by changes in the number of photosystem I complexes, which are accompanied by corresponding changes in transcripts for plastid reaction center genes. Because chloroplast protein complexes contain also many nuclear encoded components we analyzed the impact of such photosynthetic redox signals on nuclear genes. Light shift experiments combined with application of the electron transport inhibi- The light environment of plants is highly variable. This is of particular importance for photosynthesis, because changes in incident light intensity or quality can reduce the efficiency of photosynthetic electron transport and therefore the net energy fixation. Plants have developed many acclimatory mechanisms at the molecular level that enable them to cope with such changes. Most prominent responses are dynamic changes in the structure and composition of the photosynthetic apparatus (1-3).Light quality and quantity gradients that occur e.g. in dense plant populations induce an imbalance in excitation energy distribution between the two photosystems (which work electrochemically in series) and therefore reduce photosynthetic efficiency. To counteract such imbalances plants re-distribute light energy in a short term by state transitions (4, 5) and in a long term by a re-adjustment of photosystem stoichiometry. This results in a supply of more light quanta to the less active side of the electron transport chain (6 -8). Both processes are regulated by light-induced changes in the redox state of photosynthetic components (9 -11). While the short term response acts via post-translational phosphorylation of existing antenna proteins, the long term response (LTR) 1 requires the synthesis of new components and hence has to affect gene expression. This implies signaling routes that connect photosynthetic electron transport/efficiency with the expression machinery. Studies in the last decade show that such functional connections exist at multiple levels and in virtually all classes of photosynthetic organisms. In higher plants photosynthetic redox control has been found at the levels of transcription (12-19), transcript stability (20 -23), ribosome loading (24 -26), translation initiation (27), and protein accumulation (28).The origin of the respective signal transduction pathways can be very different. To date three classes of redox signals can be distinguished: the first one is generated directly within the electron transport chain, the second is represented by photosynthesis-coupled redox-active compounds such as thioredoxin or glutathione, and the third is constituted by reactive oxygen species, which are unavoidable by-products of photo...

Photosynthetic organisms acclimate to long term changes in the environmental light quality by an adjustment of their photosystem stoichiometry to maintain photosynthetic efficiency. By using light sources that predominantly excite either photosystem I (PSI) or photosystem II (PSII), we studied the effects of excitation imbalances between both photosystems on nuclear PSI gene transcription in transgenic tobacco seedlings with promoter::␤-glucuronidase gene fusions. Shifts from PSI to PSII light sources (and vice versa) induced changes in the reduction/oxidation state of intersystem redox components, and acclimation of tobacco seedlings to such changes were monitored by changes in chlorophyll a/b ratios and in vivo chlorophyll a fluorescence. The ferredoxin-NADP ؉ -oxidoreductase gene promoter did not respond to these treatments, those from the genes for subunits PsaD and PsaF of PSI are activated by a reduction signal, and the plastocyanin promoter responded to both reduction and oxidation signals. Additional experiments with photosynthetic electron transport inhibitors 3-(3,4-dichlorophenyl)-1,1-dimethyl urea and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone demonstrated that the redox state of the plastoquinone pool controls the activity of the plastocyanin promoter, whereas subunit PsaD and PsaF gene transcription is regulated by other photosynthesis-derived signals. Thus, the expression of nuclear-encoded PSI genes is controlled by diverse light quality-dependent redox signals from the plastids during photosystem stoichiometry adjustment.The photosynthetic apparatus in the chloroplasts of higher plants and algae is comprised of a patchwork of nuclear-and chloroplast-encoded components. Nuclear-encoded genes for structural components of the photosynthetic machinery are either new or a result of a gene transfer from the endosymbiotic ancestor of chloroplasts to the nucleus of the host cell (1-4). The enormous differences in gene copy number between both compartments require a highly coordinated regulation in their expression during development and acclimation of the organism to environmental cues. This coordination is controlled by the nucleus at many levels (5) but also involves signals from the plastids, which influence the expression of nuclear genes for plastid proteins (6 -9). The exact nature of the plastidderived signal(s) is still elusive. Inhibition of either plastid transcription or translation or photo-oxidative destruction of chloroplasts prevents the transcription of several nuclearencoded photosynthesis genes (10 -13). Other crucial components involved in this interorganelle cross-talk are intermediates and/or components of the tetrapyrrol biosynthesis pathway (14 -18) or the availability of phosphoenolpyruvate (19). One of the central players in this scenario is light, and it seems to regulate the expression of several nuclear genes for plastid proteins via the same cis-active elements as the plastidderived signal(s) (20). Besides cytosolic photoreceptors plants sense changes in light quantity ...

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