Photo and nutritional regulation of chloroplast valyl-tRNA synthetase in Euglena

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.

Overproduction by gene amplification of the multifunctional arom protein confers glyphosate tolerance to a plastid-free mutant of Euglena gracilis

Cells of the plastid-free mutant line of Euglena gracilis var. bacillaris, W10BSmL, can be adapted to glyphosate [N-(phosphonomethyl)glycine] by gradually increasing the concentration of the herbicide in the culture medium. The molecular basis of glyphosate tolerance is the selective ca. ten-fold overproduction of the multifunctional arom protein catalyzing steps 2-6 in the pre-chorismate pathway. Determination of 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase (E.C.2.5.1.19), shikimate:NADP+ oxidoreductase (E.C.1.1.1.25) and shikimate kinase (E.C.2.7.1.71) activities after non-denaturing gel electrophoresis, in combination with two-dimensional separations, revealed an increase in all three enzyme activities associated with overproduction of a 165 kDa protein in cells adapted to 6 mM glyphosate. Further evidence for an involvement of the multifunctional arom protein in aromatic amino acid synthesis in the plastid-free W10BSmL cells was obtained by Northern hybridization with ARO1-, aroA-, aroL- and aroE-specific Saccharomyces cerevisiae gene probes encoding the entire arom protein or parts of the EPSP synthase, shikimate:NADP+ oxidoreductase and shikimate kinase domains, respectively. Overproduction in adapted relative to control cells of a 5.3 kb transcript that cross-hybridized with all of the different probes could be demonstrated. The elevated content of the arom transcript correlated with a selective amplification of two out of five genomic sequences that hybridized with the S. cerevisiae ARO1 gene probe in Southern blots. One of the amplified genomic fragments is assumed to encode the previously identified monofunctional 59 kDa EPSP synthase, which is thought to be an organellar protein, that accumulates to a certain extent in its enzymatically active precursor form of 64.5 kDa in the plastid-free W10BSmL cells.

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.

Photocontrol of organelle and cell type specific changes in the polypeptide composition of Euglena and sorghum

Two-dimensional (2-D) gel electrophoresis has been used to follow changes in cell type specific and organelle localized polypeptides upon exposure of etiolated sorghum shoots and dark-grown resting Euglena to light. Total protein extracted from isolated bundle sheath strands and mesophyll protoplasts was resolved by 2-D gel electrophoresis. The cell type specific polypeptides were localized on the whole shoot 2-D gel map in order to determine changes in the levels of these polypeptides upon light exposure. An image analyzer was used to analyze fluorographs of 2-D gels of total Euglena protein pulse-labeled with [35S]sulfate in the dark, immediately upon light exposure and 1, 4, 14, 24, 48 and 72 h after light exposure. The subset of polypeptides whose relative rate of synthesis changes more than threefold immediately upon light exposure was identified. The different patterns of changes in the rate of synthesis of this subset of polypeptides were followed.

Photocontrol of the polypeptide composition ofEuglena : Analysis by two-dimensional gel electrophoresis

Two-dimensional gel electrophoresis resolved total cellular protein fromEuglena gracilis Klebs var.bacillaris Cori into 650 polypeptides detectable by silver staining. Exposure of dark-grown restingEuglena to light for 72 h increased the relative amounts of 79 polypeptides and decreased the relative amounts of 72 polypeptides. Four polypeptides whose level increased upon light exposure were undetectable in the bleached mutant W3BUL. Since W3BUL has lost most if not all of its chloroplast genome, these polypeptides may be coded by the chloroplast genome. Although the majority of the polypeptides studied appear to be coded by the nuclear or mitochondrial genomes, seven polypeptides which were present in W3BUL were not detectable in another chloroplast-DNA-deficient, bleached mutant, W10BSmL. It appears that a number of nonchloroplast genes are no longer expressed in this mutant. Exposure of dark-grown resting cells of the bleached mutant, W3BUL, to light increased the relative amounts of 12 polypeptides and decreased the relative amounts of 14 polypeptides. Since W3BUL lacks detectable protochlorophyll(ide), the chloroplast photoreceptor, the levels of these polypeptides are regulated by light acting through a nonchloroplast photoreceptor. Exposure of cells to light had no detectable effect on the relative amounts of those polypeptides which were present in W10BSmL, even though W10BSmL has a nonchloroplast photoreceptor. EitherEuglena contains multiple nonchloroplast photoreceptors, some of which are absent from W10BSmL, or the nonchloroplast photoreceptor present in W10BSmL is uncoupled from some of the events normally controlled by that photoreceptor.