The evolution of light stress proteins in photosynthetic organisms

The Elip (early light-inducible protein) family in pro- and eukaryotic photosynthetic organisms consists of more than 100 different stress proteins. These proteins accumulate in photosynthetic membranes in response to light stress and have photoprotective functions. At the amino acid level, members of the Elip family are closely related to light-harvesting chlorophyll a/b-binding (Cab) antenna proteins of photosystem I and II, present in higher plants and some algae. Based on their predicted secondary structure, members of the Elip family are divided into three groups: (a) one-helix Hlips (high light-induced proteins), also called Scps (small Cab-like proteins) or Ohps (one-helix proteins); (b) two-helix Seps (stress-enhanced proteins); and (c) three-helix Elips and related proteins. Despite having different physiological functions it is believed that eukaryotic three-helix Cab proteins evolved from the prokaryotic Hlips through a series of duplications and fusions. In this review we analyse the occurrence of Elip family members in various photosynthetic prokaryotic and eukaryotic organisms and discuss their evolutionary relationship with Cab proteins.     

Genome-wide analysis of the family of light-harvesting chlorophyll a/b-binding proteins in arabidopsis and rice

Light-harvesting antenna system possesses an inherent property of photoprotection. The single-helix proteins found in cyanobacteria play role in photoprotection and/or pigment metabolism. The photoprotective functions are also manifested by the two- and four-helix proteins. The photoprotection mechanism evolved earlier to the mechanism of light-harvesting of the antenna complex. Here, the light-harvesting complex genes of photosystems I and II from Arabidopsis are enlisted, and almost similar set of genes are identified in rice. Also, the three-helix early light-inducible proteins (ELIPs), two-helix stress-enhanced proteins (SEPs) and one-helix high light-inducible proteins [one-helix proteins (OHPs)] are identified in rice. Interestingly, two independent genomic loci encoding PsbS protein are also identified with implications on additional mode of non-photochemical quenching (NPQ) mechanism in rice. A few additional LHC-related genes are also identified in rice (LOC_Os09g12540, LOC_Os02g03330). This is the first report of identification of light-harvesting complex genes and light-inducible genes in rice.Key words: Lhca and Lhcb proteins, Lhc proteins evolution, light-inducible proteins, protein alignment, PsbSThe light-harvesting proteins are present in different taxa. The proteins of light-harvesting systems from higher plants, cyano-bacteria, purple bacteria and green sulphur bacteria share no sequence similarity however little structural similarity can be seen.¹ Apparently, the light-harvesting systems in these different taxa might have evolved independently from each other.¹ To enable efficient transfer of excitation energy into the reaction centers, where charge separation takes place, different proteins are recruited in order to coordinate the photosynthetic pigment molecules. The light-harvesting and light dissipation are tightly coupled processes involving the higher plant light-harvesting antenna. Here, genome-wide analysis of the light-harvesting chlorophyll a/b-binding proteins and light-inducible proteins in Arabidopsis thaliana L. and Oryza sativa L. (rice) is conducted. This study wherein genes coding for antenna proteins are identified and named can be used as a nomenclature guide to the light-harvesting complex gene family members and their relatives in rice.     

Stable insertion of the early light-induced proteins into etioplast membranes requires chlorophyll a

Etiolated plant seedlings exposed to light respond by transient accumulation of the nucleus-encoded, plastid-located early light-inducible proteins (Elips). These proteins are distant relatives of the light-harvesting chlorophyll a/b-binding gene family and bind pigments with unusual characteristics. To investigate whether accumulation of Elips in plastid membranes is post-translationally regulated by pigments, reconstitution studies were performed, where in vitro transcribed and translated low molecular mass Elip precursors of barley were combined with lysed barley etioplasts complemented with various compositions of isolated pigments. We showed that the membrane insertion of Elips, as proven by protease protection assays and washes with a chaotropic salt or alkali, depended strictly on chlorophyll a but not on chlorophyll b or xanthophyll zeaxanthin. The amount of inserted Elips increased almost linearly with the chlorophyll a concentration, and the insertion efficiency was not significantly influenced by a light intensity between 1 and 1,000 micromol x m(-2) x s(-1). In contrast, in vitro import of Elip precursors into greening plastids was enhanced by high intensity light. Thus, we conclude that although chlorophylls bound to Elips seem to not be involved in light harvesting, they are crucial for a stable insertion of these proteins into the plastid membrane.     

Photosynthetic capacity and light harvesting efficiency during the winter-to-spring transition in subalpine conifers

Some coniferous forest ecosystems undergo complete photosynthetic down-regulation in winter. The present study examined the influence of several environmental parameters on intrinsic, needle-level photosynthesis and photoprotection during the spring reactivation of photosynthesis in subalpine conifers. Maximal photosystem II (PSII) efficiency, photosynthetic capacity, and amounts of zeaxanthin and early light-inducible protein (Elip) family members were assessed in three subalpine conifer species over 3 years, and intensively during the 2003 winter-to-spring transition. During summers, maximal PSII efficiency remained high while intrinsic photosynthetic capacity varied depending on precipitation. During winters and the winter-to-spring transition, photosynthetic capacity and PSII efficiency were highly correlated and (during the spring transition) strongly influenced by air and soil temperature and liquid water availability. Decreases in the amount of Elip family members from winter through spring paralleled disengagement of sustained zeaxanthin-dependent photoprotection, although one of four anti-Elip antibody-reactive bands increased during spring. Intrinsic photosynthetic capacity and maximal PSII efficiency were highly responsive to day-to-day environmental changes during spring, indicating that multiple environmental signals are integrated to orchestrate the reactivation of photosynthesis from the inactive winter state to the active summer state.     

Tetrapyrrole involvement in expression regulation of a nuclear gene of low-molecular-weight plastid protein ELIP

An inhibitor analysis was used for studying the tetrapyrrole role in the regulation of the expression of the nuclear gene encoding a low-molecular-weight protein, a stress plastid light-inducible protein ELIP. 2,2'-Dipyridyl and norflurazon were used as inhibitors. Experiments with dipyridyl demonstrated that tetrapyrroles were involved in the regulation of Elip gene expression, inhibiting it by approximately 50%. Similar results were obtained when there was photodestruction of the chloroplasts, caused by a plant treatment with norflurazon. The results confirm the involvement of the chloroplasts in the regulation of the nuclear gene expression coding for plastid proteins. Tetrapyrroles are important contributors to this process.     

Winter down-regulation of intrinsic photosynthetic capacity coupled with up-regulation of Elip-like proteins and persistent energy dissipation in a subalpine forest

Overwintering, sun-exposed and photosynthetically inactive evergreens require powerful photoprotection. The goal of this study was to seasonally characterize photosynthesis and key proteins/components involved in electron transport and photoprotection. Maximal photosystem II (PSII) efficiency and photosynthetic capacity, amounts of zeaxanthin (Z), antheraxanthin (A), pheophytin and proteins (oxygen-evolving 33 kDa protein (OEC), PSII core protein D1 and subunit S (PsbS) protein, and members of the early light-inducible protein (Elip) family) were assessed in five conifer species at high altitude and in ponderosa pine (Pinus ponderosa) at moderate altitude during summer and winter. Relative to summer, winter down-regulation of photosynthetic capacity and loss of PSII efficiency at the high-altitude sites were paralleled by decreases in OEC, D1, and pheophytin; massive nocturnal retention of (Z + A) and up-regulation of two to four proteins cross-reactive with anti-Elip antibodies; and no change in PsbS amount. By contrast, ponderosa pine at moderate altitude exhibited no down-regulation of photosynthetic capacity, smaller depressions in PSII efficiency, and less up-regulation of Elip family members. These results support a function for members of the Elip family in the acclimation of sun-exposed needles that down-regulate photosynthesis during winter. A possible role in sustained photoprotection is considered.     

Circadian Control of the Accumulation of mRNAs for Light- and Heat-Inducible Chloroplast Proteins in Pea (Pisum sativum L.)

The levels of the mRNAs for light-inducible, nuclear-coded chloroplast proteins vary rhythmically in pea (Pisum sativum L.) plants either grown in a dark-light cycle or under constant light conditions. This has been observed for the early light-inducible protein, the light-harvesting chlorophyll a/b protein, and the small subunit of the ribulose-1,5-bisphosphate carboxylase. The mRNA levels are high in the morning, exhibit a minimum in the first half of the night, and increase again during the second half of the night. The amplitude of fluctuation is between 5- and 10-fold. A similar change in the mRNA abundance was found for four nuclear encoded heat-shock proteins of 18, 24, 26, and 30 kilodaltons. The ability of plants to transcribe heat-shock genes upon heat-shock for 2 hours varies through the day. The maxima for induction are found in the second half of the night and the morning. The minima are reached during the afternoon. The degree of fluctuation is between 3- and 5-fold. The levels of mRNAs for cytosolic as well as for plastid heat-shock proteins oscillate in parallel.     

High-Mobility-Group A-Like CarD Binds to a DNA Site Optimized for Affinity and Position and to RNA Polymerase To Regulate a Light-Inducible Promoter in Myxococcus xanthus

The CarD-CarG complex controls various cellular processes in the bacterium Myxococcus xanthus including fruiting body development and light-induced carotenogenesis. The CarD N-terminal domain, which defines the large CarD_CdnL_TRCF protein family, binds to CarG, a zinc-associated protein that does not bind DNA. The CarD C-terminal domain resembles eukaryotic high-mobility-group A (HMGA) proteins, and its DNA binding AT hooks specifically recognize the minor groove of appropriately spaced AT-rich tracts. Here, we investigate the determinants of the only known CarD binding site, the one crucial in CarD-CarG regulation of the promoter of the carQRS operon (P_QRS), a light-inducible promoter dependent on the extracytoplasmic function (ECF) σ factor CarQ. In vitro, mutating either of the 3-bp AT tracts of this CarD recognition site (TTTCCAGAGCTTT) impaired DNA binding, shifting the AT tracts relative to P_QRS had no effect or marginally lowered DNA binding, and replacing the native site by the HMGA1a binding one at the human beta interferon promoter (with longer AT tracts) markedly enhanced DNA binding. In vivo, however, all of these changes deterred P_QRS activation in wild-type M. xanthus, as well as in a strain with the CarD-CarG pair replaced by the Anaeromyxobacter dehalogenans CarD-CarG (CarD_Ad-CarG_Ad). CarD_Ad-CarG_Ad is functionally equivalent to CarD-CarG despite the lower DNA binding affinity in vitro of CarD_Ad, whose C-terminal domain resembles histone H1 rather than HMGA. We show that CarD physically associates with RNA polymerase (RNAP) specifically via interactions with the RNAP β subunit. Our findings suggest that CarD regulates a light-inducible, ECF σ-dependent promoter by coupling RNAP recruitment and binding to a specific DNA site optimized for affinity and position.     

Differential Control of Xanthophylls and Light-Induced Stress Proteins, as Opposed to Light-Harvesting Chlorophyll a/b Proteins, during Photosynthetic Acclimation of Barley Leaves to Light Irradiance

Barley (Hordeum vulgare L.) plants were grown at different photon flux densities ranging from 100 to 1800 μmol m⁻² s⁻¹ in air and/or in atmospheres with reduced levels of O₂ and CO₂. Low O₂ and CO₂ partial pressures allowed plants to grow under high photosystem II (PSII) excitation pressure, estimated in vivo by chlorophyll fluorescence measurements, at moderate photon flux densities. The xanthophyll-cycle pigments, the early light-inducible proteins, and their mRNA accumulated with increasing PSII excitation pressure irrespective of the way high excitation pressure was obtained (high-light irradiance or decreased CO₂ and O₂ availability). These findings indicate that the reduction state of electron transport chain components could be involved in light sensing for the regulation of nuclear-encoded chloroplast gene expression. In contrast, no correlation was found between the reduction state of PSII and various indicators of the PSII light-harvesting system, such as the chlorophyll a-to-b ratio, the abundance of the major pigment-protein complex of PSII (LHCII), the mRNA level of LHCII, the light-saturation curve of O₂ evolution, and the induced chlorophyll-fluorescence rise. We conclude that the chlorophyll antenna size of PSII is not governed by the redox state of PSII in higher plants and, consequently, regulation of early light-inducible protein synthesis is different from that of LHCII.     

DAZ Family Proteins,Key Players for Germ Cell Development

DAZ family proteins are found almost exclusively in germ cells in distant animal species. Deletion or mutations of their encoding genes usually severely impair either oogenesis or spermatogenesis or both. The family includes Boule (or Boll), Dazl (or Dazla) and DAZ genes. Boule and Dazl are situated on autosomes while DAZ, exclusive of higher primates, is located on the Y chromosome. Deletion of DAZ gene is the most common causes of infertility in humans. These genes, encoding for RNA binding proteins, contain a highly conserved RNA recognition motif and at least one DAZ repeat encoding for a 24 amino acids sequence able to bind other mRNA binding proteins. Basically, Daz family proteins function as adaptors for target mRNA transport and activators of their translation. In some invertebrate species, BOULE protein play a pivotal role in germline specification and a conserved regulatory role in meiosis. Depending on the species, DAZL is expressed in primordial germ cells (PGCs) and/or pre-meiotic and meiotic germ cells of both sexes. Daz is found in fetal gonocytes, spermatogonia and spermatocytes of adult testes. Here we discuss DAZ family genes in a phylogenic perspective, focusing on the common and distinct features of these genes, and their pivotal roles during gametogenesis evolved during evolution.     

Hydroxysteroid dehydrogenase family proteins on lipid droplets through bacteria, C. elegans, and mammals

Lipid droplets (LDs) are the main fat storing sites in almost all species from bacteria to humans. The perilipin family has been found as LD proteins in mammals, Drosophila, and a couple of slime molds, but no bacterial LD proteins containing sequence conservation were identified. In this study, we reported that the hydroxysteroid dehydrogenase (HSD) family was found on LDs across all organisms by LD proteomic analysis. Imaging experiments confirmed LD targeting of three representative HSD proteins including ro01416 in RHA1, DHS-3 in C. elegans, and 17β-HSD11 in human cells. In C. elegans, 17β-HSD11 family proteins (DHS-3, DHS-4 and DHS-19) were localized on LDs in distinct tissues. In intestinal cells of C. elegans, DHS-3 targeted to cytoplasmic LDs, while DHS-9 labeled nuclear LDs. Furthermore, the N-terminal hydrophobic domains of 17β-HSD11 family were necessary for their targeting to LDs. Last, 17β-HSD11 family proteins induced LD aggregation, and deletion of DHS-3 in C. elegans caused lipid decrease. Independent of their presumptive catalytic sites, 17β-HSD11 family proteins regulated LD dynamics and lipid metabolism through affecting the LD-associated ATGL, which was conserved between C. elegans and humans. Together, these findings for HSDs provide a new insight not only into the mechanistic studies of the dynamics and functions of LDs in multiple organisms, but also into understanding the evolutionary history of the organelle.     

Sequence conservation of light-harvesting and stress-response proteins in relation to the three-dimensional molecular structure of LHCII

The structure of pea light-harvesting complex LHCII determined to 3.4 Å resolution by electron crystallography (Kühlbrandt, Wang and Fujiyoshi (1994) Nature 367: 614–621) was examined to determine the relationship between structural elements and sequence motifs conserved in the extended family of light-harvesting antennas (Chl a/b, fucoxanthin Chl a/c proteins) and membrane-intrinsic stress-induced proteins (ELIPs) to which LHCII belongs. It is predicted that the eukaryotic ELIPs can bind at least four molecules of Chl. The one-helix prokaryotic ELIP of Synechococcus was modelled as a homodimer based on the high degree of conservation of residues involved in the interactions of the first (B) and third (A) helices of LHCII.Abbreviations CAB Chl a/b-binding - ELIP early light-inducible protein - FCP fucoxanthin-Chl a/c protein - Lut1, Lut2 lutein molecules 1 and 2     

Cloning and characterization of the HLIP gene encoding high light-inducible protein from Porphyra yezoensis

High light-inducible proteins are critical for photosynthetic organisms when responding to high light stress. We cloned and characterized the HLIP gene from Porphyra yezoensis, which was designated as PyHLIP. Sequence analysis revealed that the open reading frame of PyHLIP was 387?bp in length that encoded a 128 amino acid-polypeptide containing a plastid transit peptide and an obvious membrane-spanning α-helix region. No intron existed in the genomic DNA of PyHLIP. The putative promoter of PyHLIP was TATA-less, and the initiator element (Inr) existed upstream of ATG. Analysis of genomic organization indicated there were two copies of PyHLIP gene in the P. yezoensis nuclear genome. The expression of PyHLIP can be induced by various stress conditions and is especially sensitive to high light.