Translational regulation of protein synthesis during light-induced chloroplast development in Euglena

Proteomic Responses of Dark-Adapted Euglena gracilis and Bleached Mutant Against Light Stimuli

Euglena gracilis (E. gracilis) has secondary endosymbiotic chloroplasts derived from ancient green algae. Its chloroplasts are easily lost under numerous conditions to become permanently bleached mutants. Green cells adapted in the dark contain undeveloped proplastids and they will develop into mature chloroplasts after 3 days of light exposure. Thus, E. gracilis is an ideal model species for a chloroplast development study. Previous studies about chloroplast development in E. gracilis focused on morphology and physiology, whereas few studies have addressed the regulatory processes induced by light in the proteome. In this study, the whole-genome proteome of dark-adapted E. gracilis (WT) and permanently ofloxacin-bleached mutant (B2) was compared under the light exposure after 0, 12, and 72 h. The results showed that the photosynthesis-related proteins were up-regulated over time in both WT and B2. The B2 strain, with losing functional chloroplasts, seemed to possess a complete photosynthetic function system. Both WT and B2 exhibited significant light responses with similar alternation patterns, suggesting the sensitive responses to light in proteomic levels. The main metabolic activities for the utilization of carbon and energy in WT were up-regulated, while the proteins with calcium ion binding, cell cycle, and non-photosynthetic carbon fixation were down-regulated in B2. This study confirmed light-induced chloroplast development in WT from dark, and also for the first time investigates the light responses of a bleached mutant B2, providing more information about the unknown functions of residual plastids in Euglena bleached mutants.

Molecular tools and applications of Euglena gracilis: From biorefineries to bioremediation

Euglena gracilis is a promising source of commercially important metabolites such as vitamins, wax esters, paramylon, and amino acids. However, the molecular tools available to create improved Euglena strains are limited compared to other microorganisms that are currently exploited in the biotechnology industry. The complex poly-endosymbiotic nature of the Euglena genome is a major bottleneck for obtaining a complete genome sequence and thus represents a notable shortcoming in gaining molecular information of this organism. Therefore, the studies and applications have been more focused on using the wild-type strain or its variants and optimizing the nutrient composition and cultivation conditions to enhance the production of biomass and valuable metabolites. In addition to producing metabolites, the E. gracilis biorefinery concept also provides means for the production of biofuels and biogas as well as residual biomass for the remediation of industrial and municipal wastewater. Using Euglena for bioremediation of environments contaminated with heavy metals is of special interest due to the strong ability of the organism to accumulate and sequester these compounds. The published draft genome and transcriptome will serve as a basis for further molecular studies of Euglena and provide a guide for the engineering of metabolic pathways of relevance for the already established as well as novel applications.

Photo and Nutritional Regulation of Euglena Organelle Development

Euglena can use light and CO₂, photosynthesis, as well as a large variety of organic molecules as the sole source of carbon and energy for growth. Light induces the enzymes, in this case an entire organelle, the chloroplast, that is required to use CO₂ as the sole source of carbon and energy for growth. Ethanol, but not malate, inhibits the photoinduction of chloroplast enzymes and induces the synthesis of the glyoxylate cycle enzymes that comprise the unique metabolic pathway leading to two carbon, ethanol and acetate, assimilation. In resting, carbon starved cells, light mobilizes the degradation of the storage carbohydrate paramylum and transiently induces the mitochondrial proteins required for the aerobic metabolism of paramylum to provide the carbon and energy required for chloroplast development. Other mitochondrial proteins are degraded upon light exposure providing the amino acids required for the synthesis of light induced proteins. Changes in protein levels are due to increased and decreased rates of synthesis rather than changes in degradation rates. Changes in protein synthesis rates occur in the absence of a concomitant increase in the levels of mRNAs encoding these proteins indicative of photo and metabolic control at the translational rather than the transcriptional level. The fraction of mRNA encoding a light induced protein such as the light harvesting chlorophyll a/b binding protein of photosystem II, (LHCPII) associated with polysomes in the dark is similar to the fraction associated with polysomes in the light indicative of photoregulation at the level of translational elongation. Ethanol, a carbon source whose assimilation requires carbon source specific enzymes, the glyoxylate cycle enzymes, represses the synthesis of chloroplast enzymes uniquely required to use light and CO₂ as the sole source of carbon and energy for growth. The catabolite sensitivity of chloroplast development provides a mechanism to prioritize carbon source utilization. Euglena uses all of its resources to develop the metabolic capacity to utilize carbon sources such as ethanol which are rarely in the environment and delays until the rare carbon source is no longer available forming the chloroplast which is required to utilize the ubiquitous carbon source, light and CO₂.

Translational regulation in chloroplasts for development and homeostasis

Chloroplast genomes encode 100-200 proteins which function in photosynthesis, the organellar genetic system, and other pathways and processes. These proteins are synthesized by a complete translation system within the chloroplast, with bacterial-type ribosomes and translation factors. Here, we review translational regulation in chloroplasts, focusing on changes in translation rates which occur in response to requirements for proteins encoded by the chloroplast genome for development and homeostasis. In addition, we delineate the developmental and physiological contexts and model organisms in which translational regulation in chloroplasts has been studied. This article is part of a Special Issue entitled: Chloroplast biogenesis.

Translational regulation of plastid gene expression in Euglena gracilis

Translation of plastid messenger RNAs depends on aminoacyl-tRNAs formed by charging plastid-encoded tRNAs with cognate amino acids. The enzymes involved, chloroplast aminoacyl-tRNA synthetases, are encoded in the nucleus. Both the tRNAs and the aminoacyl-tRNA synthetases are stimulated in synthesis if dark-grown cells are exposed to light. However, their accumulation during light-induced chloroplast development in Euglena gracilis starts with an appreciable lag-phase. During this period the availability of charged tRNAs probably limits protein synthesis. Due to the contemporary need of glutamyl-tRNAGluGAA in chlorophyll synthesis this particular tRNA is very likely depleted. Based on an analysis of glutamate codon frequency in known plastid genes, the effect of a glutamyl-tRNAGluGAA limitation on the translation of plastid messages is discussed.

In-vitro transport of chloroplast proteins in a homologousEuglena system with particular reference to plastid leucyl-tRNA synthetase

In-vitro translations of total or polyadenylated RNA from chemoorganotrophic and photoautotrophicEuglena gracilis showed no substantial differences in the polypeptide patterns of the two cell types. By contrast, the corresponding patterns of in-vivo labelling indicated that posttranscriptional control of abundant cellular proteins occurred in illuminated cells. This type of control was confirmed forEuglena chloroplast proteins of cytoplasmic origin. The posttranslational transport of in-vitro-formed polypeptides into homologous chloroplasts allowed the plastid-targeted proteins to be recognized. Estimations of the amounts of in-vitro-translated polypeptides showed that the mRNA levels for nuclearencoded chloroplast proteins were almost constant throughout chloroplast development inEuglena. The import of the in-vitro-translation products into the chloroplasts was demonstrated (i) by kinetic determination of radioactivity increasing inside and decreasing outside the organelles, (ii) by autoradiographic analysis of the transported translation products among two-dimensionally separated chloroplast proteins, and (iii) by autoradiographic estimation of the decrease in labelled polypeptides in the incubation medium. After 60 min of incubation in the presence of chloroplasts, about 20% of the in-vitro-labelled translation products, corresponding to 30 protein spots, were found to be redistributed into the organelles. Low-abundance chloroplast leucyl-tRNA synthetase (LeuRS) was detected immunologically among the in-vitro translation products as a precursor protein (pre-LeuRS) of 112 kilodaltons (kDa), which was transported into chloroplasts where it was present in the form of the 105-kDa mature enzyme. Processing of pre-LeuRS also seemed to occur using wheat-germ translation system; however, the mature enzyme was not sequestered intoEuglena chloroplasts. Our results indicate that a nuclear-encoded chloroplastic aminoacyl-tRNA synthetase, the product of a low-abundance mRNA, is transported into and processed in chloroplasts in vitro.

Synthesis of EF-Tu and distribution of its mRNA between stroma and thylakoids during the cell cycle of Chlamydomonas reinhardii

In Chlamydomonas reinhardii the elongation factor EF-Tu is encoded in the chloroplast DNA. We identified EF-Tu in the electrophoretic product pattern of chloroplast-made proteins and showed that this protein is only synthesized in the first half of the light period in synchronized cells. The newly synthesized EF-Tu contributed little to the almost invariable content of EF-Tu in chloroplasts during the light period of the cell cycle. However, increasing cell volume and the lack of EF-Tu synthesis in the second half of the light period led to a decrease in the concentration of EF-Tu in chloroplasts. At different times in the vegetative cell cycle, the RNA was extracted from whole chloroplasts and from free and thylakoid-bound chloroplast polysomes. The content of mRNA of EF-Tu in chloroplasts and the distribution between stroma and thylakoids were determined. During the light period, the content of the mRNA for EF-Tu varied in parallel to the rate of EF-Tu synthesis. However, in the dark, some mRNA was present even in the absence of EF-Tu synthesis. Most of the mRNA was bound to thylakoids during the whole cell cycle. This suggests that synthesis of EF-Tu is associated with thylakoid membranes.

Synthesis of two proteins in chloroplasts and mRNA distribution between thylakoids and stroma during the cell cycle of Chlamydomonas reinhardii

Chloroplasts contain thylakoid-bound and free ribosomes and polysomes. Whether binding of polysomes plays an immediate role in the regulation of chloroplast protein synthesis is not yet clear. In the present work, variations of protein synthesis and of mRNA content were measured not in greening, but in fully differentiated chloroplasts during the cell cycle of synchronized cultures of Chlamydomonas reinhardii. At different times of the vegetative cell cycle, the RNA was extracted from free and thylakoid-bound chloroplast polysomes and the partition of mRNAs between stroma and thylakoids was measured for two proteins, i.e. the 32-kDa herbicide-binding membrane protein and the soluble large subunit of the ribulose-1,5-bisphosphate carboxylase. At the same time the rates of synthesis of these two proteins were also determined. At 2 h after the onset of light, the content of both mRNAs in chloroplasts had doubled and 75-90% of each of these mRNAs were found to be bound to the thylakoids. The rate of protein synthesis, however, increased 10-fold, but reached its maximum only after about 6 h in the light. The differences in the time courses, in the stimulation of the rate of protein synthesis, and in the mRNA-binding to thylakoids point to a translational regulation of protein synthesis. Furthermore, since a very high proportion of polysomes were bound to thylakoids, containing mRNA for both a membrane and a soluble protein, this light-induced binding of polysomes to thylakoids seems to be an essential, but not the only, prerequisite for protein synthesis in chloroplasts.