Expression of the chlorophyll-a/b-protein multigene family in pea (Pisum sativum L.) : Evidence for distinct developmental responses

Elastin-Like Polypeptides for Biomedical Applications

Elastin-like polypeptides (ELPs) are stimulus-responsive biopolymers derived from human elastin. Their unique properties—including lower critical solution temperature phase behavior and minimal immunogenicity—make them attractive materials for a variety of biomedical applications. ELPs also benefit from recombinant synthesis and genetically encoded design; these enable control over the molecular weight and precise incorporation of peptides and pharmacological agents into the sequence. Because their size and sequence are defined, ELPs benefit from exquisite control over their structure and function, qualities that cannot be matched by synthetic polymers. As such, ELPs have been engineered to assemble into unique architectures and display bioactive agents for a variety of applications. This review discusses the design and representative biomedical applications of ELPs, focusing primarily on their use in tissue engineering and drug delivery.

Sulphur responsiveness of the Chlamydomonas reinhardtii LHCBM9 promoter

A 44-base-pair region in the Chlamydomonas reinhardtii LHCBM9 promoter is essential for sulphur responsiveness. The photosynthetic light-harvesting complex (LHC) proteins play essential roles both in light capture, the first step of photosynthesis, and in photoprotective mechanisms. In contrast to the other LHC proteins and the majority of photosynthesis proteins, the Chlamydomonas reinhardtii photosystem II-associated LHC protein, LHCBM9, was recently reported to be up-regulated under sulphur deprivation conditions, which also induce hydrogen production. Here, we examined the sulphur responsiveness of the LHCBM9 gene at the transcriptional level, through promoter deletion analysis. The LHCBM9 promoter was found to be responsive to sulphur deprivation, with a 44-base-pair region between nucleotide positions -136 and -180 relative to the translation start site identified as essential for this response. Anaerobiosis was found to enhance promoter activity under sulphur deprivation conditions, however, alone was unable to induce promoter activity. The study of LHCBM9 is of biological and biotechnological importance, as its expression is linked to photobiological hydrogen production, theoretically the most efficient process for biofuel production, while the simplicity of using an S-deprivation trigger enables the development of a novel C. reinhardtii-inducible promoter system based on LHCBM9.

Circadian regulation of chloroplastic f and m thioredoxins through control of the CCA1 transcription factor

Chloroplastic thioredoxins f and m (TRX f and TRX m) mediate light regulation of carbon metabolism through the activation of Calvin cycle enzymes. The role of TRX f and m in the activation of Calvin cycle enzymes is best known among the TRX family. However, the discoveries of new potential targets extend the functions of chloroplastic TRXs to other processes in non-photosynthetic tissues. As occurs with numerous chloroplast proteins, their expression comes under light regulation. Here, the focus is on the light regulation of TRX f and TRX m in pea and Arabidopsis during the day/night cycle that is maintained during the subjective night. In pea (Pisum sativum), TRX f and TRX m1 expression is shown to be governed by a circadian oscillation exerted at both the transcriptional and protein levels. Binding shift assays indicate that this control probably involves the interaction of the CCA1 transcription factor and an evening element (EE) located in the PsTRX f and PsTRX m1 promoters. In Arabidopsis, among the multigene family of TRX f and TRX m, AtTRX f2 and AtTRX m2 mRNA showed similar circadian oscillatory regulation, suggesting that such regulation is conserved in plants. However, this oscillation was disrupted in plants overexpressing CCA1 (cca1-ox) or repressing CCA1 and LHY (cca1-lhy). The physiological role of the oscillatory regulation of chloroplastic TRX f and TRX m in plants during the day/night cycle is discussed.

A role for ethylene in the phytochrome-mediated control of vegetative development

Members of the phytochrome family of photoreceptors play key roles in vegetative plant development, including the regulation of stem elongation, leaf development and chlorophyll accumulation. Hormones have been implicated in the control of these processes in de-etiolating seedlings. However, the mechanisms by which the phytochromes regulate vegetative development in more mature plants are less well understood. Pea (Pisum sativum) mutant plants lacking phytochromes A and B, the two phytochromes present in this species, develop severe defects later in development, including short, thick, distorted internodes and reduced leaf expansion, chlorophyll content and CAB gene transcript level. Studies presented here indicate that many of these defects in phyA phyB mutant plants appear to be due to elevated ethylene production, and suggest that an important role of the phytochromes in pea is to restrict ethylene production to a level that does not inhibit vegetative growth. Mutant phyA phyB plants produce significantly more ethylene than WT plants, and application of an ethylene biosynthesis inhibitor rescued many aspects of the phyA phyB mutant phenotype. This deregulation of ethylene production in phy-deficient plants appears likely to be due, at least in part, to the elevated transcript levels of key ethylene-biosynthesis genes. The phytochrome A photoreceptor appears to play a prominent role in the regulation of ethylene production, as phyA, but not phyB, single-mutant plants also exhibit a phenotype consistent with elevated ethylene production. Potential interactions between ethylene and secondary plant hormones in the control of the phy-deficient mutant phenotype were explored, revealing that ethylene may inhibit stem elongation in part by reducing gibberellin levels.

Cryptochrome 1 contributes to blue-light sensing in pea

Cryptochromes are widespread in higher plants but their physiological roles as blue-light photoreceptors have been examined in relatively few species. Screening in a phyA null mutant background has identified several blue-light response mutants in pea (Pisum sativum), including one that carries a substitution of a highly conserved glycine residue in the N-terminal photolyase-homologous domain of the pea CRY1 gene. Analyses of cry1, phyA, and phyB mutants show that all three photoreceptors contribute to seedling photomorphogenesis under high-irradiance blue light, whereas phyA is the main photoreceptor active under low irradiances. Triple phyA phyB cry1 mutants grown under high-irradiance blue light are indistinguishable from dark-grown wild-type plants in length and leaf expansion but show a small residual response to higher-irradiance white light. Monogenic cry1 mutants have little discernable phenotype at the seedling stage, but later in development are more elongated than wild-type plants. In addition, the loss of cry1 moderates the short-internode phenotype of older phyA mutants, suggesting an antagonism between phyA and cry1 under some conditions. Pea cry1 has a small inhibitory effect on flowering under long and short days. However, the phyA cry1 double mutant retains a clear promotion of flowering in response to blue-light photoperiod extensions, indicating a role for one or more additional blue-light photoreceptors in the control of flowering in pea.

Cytokinin stimulates polyribosome loading of nuclear-encoded mRNAs for the plastid ATP synthase in etioplasts of Lupinus luteus: the complex accumulates in the inner-envelope membrane with the CF(1) moiety located towards the stromal space

Three of the nine subunits of the plastid ATP synthase, including the subunit of the CF(1) moiety (gene AtpC), are encoded in the nucleus. Application of cytokinin to etiolated lupine seedlings induces polyribosome association of their mRNAs. This appears to be specific as no such regulation was observed for messages for three ribosomal proteins. Cytokinin-mediated polyribosome loading was also observed for the spinach AtpC message in etiolated transgenic tobacco seedlings. Analysis of various spinach AtpC mRNA derivatives uncovered that the 5' untranslated region (5' UTR) of this message is sufficient to direct polyribosome loading, and that sequences at the 3' end of the AtpC 5' UTR, including an UC-rich motif, are crucial for this regulation. The increase in polyribosome loading of the AtpC message correlated with an increased synthesis of the polypeptide. The subunit, together with the ATP synthase complex, accumulates in the inner-envelope membrane with the CF(1) moiety located towards the stromal space of the etioplast. These results suggest that cytokinin promotes accumulation of the ATP synthase in the inner-envelope membrane of lupine etioplasts by stimulating the translation efficiency of their nuclear-encoded messages.

Origins of phytochrome-modulated Lhcb mRNA expression in seed plants

The levels of Lhcb mRNA in higher plants are regulated by phytochrome, cryptochrome, and an endogenous circadian oscillator. To determine whether similar regulatory mechanisms operate in the ancient gymnosperm Ginkgo biloba, we measured Lhcb mRNA levels in seedlings in response to different light conditions. Removal of a diurnally oscillating light stimulus caused dampening of maximal Lhcb mRNA accumulation levels, with little change in periodicity. Although low fluence pulses of both red and blue light given to etiolated seedlings caused maximal accumulation of Lhcb mRNAs characteristic of the phasic/circadian response seen in flowering plants, the additional initial acute response seen in flowering plants was absent. The induction of Lhcb gene expression in both cases was at least partially reversible by far-red light, and appeared biphasic over a range of red fluences. Together, these data indicate that Lhcb genes in G. biloba appear to be regulated in a manner similar to that of flowering plants, whereas signaling and attenuation of mRNA levels through the photoreceptor systems and circadian clock show features distinct from those characterized to date. The implications for these findings are discussed in light of the evolution of circadian clock input signaling.

EVOLUTION OF LIGHT-REGULATED PLANT PROMOTERS

In this review, we address the phylogenetic and structural relationships between light-responsive promoter regions from a range of plant genes, that could explain both their common dependence on specific photoreceptor-associated transduction pathways and their functional versatility. The well-known multipartite light-responsive elements (LREs) of flowering plants share sequences very similar to motifs in the promoters of orthologous genes from conifers, ferns, and mosses, whose genes are expressed in absence of light. Therefore, composite LREs have apparently evolved from cis-regulatory units involved in other promoter functions, a notion with significant implications to our understanding of the structural and functional organization of angiosperm LREs.

Cloning of cDNA encoding the rice 22 kDa protein of Photosystem II (PSII-S) and analysis of light-induced expression of the gene

Cloning of rice cDNA encoding the chlorophyll-binding 22 kDa protein of Photosystem II (PSII-S) and the light-induced expression of the gene are reported. One of the light-responsive cDNA clones, isolated by screening with a light-specific subtracted cDNA probe, was shown to encode PSII-S of rice. Genomic Southern analysis suggested that the PSII-S gene, psbS, is a single-copy gene in rice. A brief exposure to red light induced a severalfold increase in the steady state level of PSII-S transcripts in etiolated seedlings. The red light effect was reversed by far-red light, suggesting involvement of phytochrome in the PSII-S gene regulation. Prolonged exposure (3 h) to blue light, however, revealed a much stronger effect than red light on the accumulation of PSII-S transcripts in the etiolated seedlings. In dark-adapted green plants, prolonged exposure to blue light induced re-accumulation of transcripts encoding PSII-S, whereas red light had little effect.

Different sequences for 5'-untranslated leaders of nuclear genes for plastid proteins affect the expression of the beta-glucuronidase gene

Expression of chimeric uidA gene fusions (for bacterial beta-glucuronidase) with 5'-flanking sequences of the spinach AtpC and PetE genes (encoding the subunit gamma of the chloroplast ATP synthase and plastocyanin, respectively) requires sequences for the 5'-untranslated leaders. The sequence for the PetE leader does not exhibit significant similarities to those of other leader sequences. Closer inspection of PetE uncovered that the crucial region is located in the vicinity of the transcription start site (+5/+15, TTGTCATTTCT). In contrast, 3' deletions of sequences for the AtpC leader revealed that the region in the vicinity of the translation initiation codon is essential for uidA gene expression (+103/+176). This segment contains a CT-rich sequence (TTCTCTCTCCT), which is found identically or in a slightly modified form in sequences for 85 plant leaders deposited in the EMBL data bank. Site-directed mutagenesis of the CT-rich sequence resulted in a three-fold reduction of the transcription of the transgene. It is concluded (1) that different elements in the sequences for the spinach PetE and AtpC leaders control the expression of the uidA gene, (2) that these elements operate transcriptionally rather than post-transcriptionally and (3) that a CT-rich sequence represents a crucial cis element for the transcription of the AtpC::uidA gene fusion.

Light-induced biogenesis of the light-harvesting complexes of Photosystems I and II : Gene expression and protein accumulation

The light-harvesting complexes of Photosystems I and II contain multiple chlorophyll-carotenoid-binding proteins. The stoichiometry and topology of the LHCs is precisely defined to optimally funnel captured light energy to the reaction center. The manner in which this exact arrangement is accomplished is not known. As an initial means to understand the mechanisms involved in establishing a functional LHC, the influence of light on LHC gene expression and protein accumulation was studied during the light-induced greening of etiolated wild type and chlorophyll b-less mutant barley seedlings. Light, involving phytochrome, promotes the expression of all LHC genes with the same relative kinetics. LHC protein accumulation closely parallels the increases observed in transcript levels. Differential accumulation of LHC transcripts or protein was not evident in wild type seedlings. Post-translational factors are likely to be involved in fine tuning the position and stoichiometry of the individual LHCs around the reaction center.

The light-harvesting chlorophyll a-c-binding protein of dinoflagellates: a putative polyprotein

The principle light-harvesting chlorophyll a-c-binding protein of Amphidinium carterae of 19 kDa is encoded as a polyprotein translated from a 6.1 kb mRNA. The cDNA sequences indicate that each derived polypeptide is contiguous with the next and that the mature peptides are formed by cleavage at a C-terminal arginine residue. Comparison of the amino-acid sequences shows the Amphidinium protein to be most closely related to the fucoxanthin-chlorophyll-protein (Fcp) of Phaeodactylum and less related to the chlorophyll a-b-binding (Cab) proteins including those from Euglena.

Sequence of Lhcb3*1, a gene encoding a Photosystem II chlorophyll a/b-binding protein in Pisum

We have cloned and sequenced a pea Lhcb3 gene, encoding a Photosystem II chlorophyll a/b-binding protein. Sequence analysis indicates that the gene contains two introns and predicts a polypeptide of 265 amino acids. The predicted polypeptide sequence is highly homologous to the polypeptide sequences deduced from Lhcb3 genes previously characterized in tomato and barley.

Concerted circadian oscillations in transcript levels of nineteen Lha/b (cab) genes in Lycopersicon esculentum (tomato)

Steady-state mRNA levels of nineteen members of the Lha/b (cab) gene family of Lycopersicon esculentum, encoding nine different types of light-harvesting complex (LHC) polypeptides, were determined by primer extension analysis. Each Lha/b gene is expressed and individual mRNAs accumulate to distinct levels. The relative contribution of each Lha/b mRNA to the total Lha/b mRNA levels is very similar in different green organs (leaves, stems, fruits, sepals) and after light treatment of etiolated seedlings. Detailed analysis of Lha/b mRNA accumulation in leaves under light/dark conditions, continuous darkness and continuous light revealed diurnal and circadian oscillations of Lha/b mRNAs for all genes. Only minor instances of divergence from a general expression pattern are apparent. Together these results indicate a concerted expression of all genes, suggesting that similar or identical molecular mechanisms and signal transduction chain control the expression of all Lha/b genes.