Poly(adenylic acid)-containing RNA of Euglena gracilis during chloroplast development. I. Analysis of their complexity by hybridization to complementary DNA

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.

Catabolite repression of chloroplast development in Euglena

Two-dimensional gel electrophoresis resolved total cellular protein from Euglena gracilis Klebs var. bacillaris Cori into 640 polypeptides detectable by silver staining. The addition of 84 mM ethanol to dark-grown resting carbon-starved cells increased the relative amounts of 6 polypeptides and decreased the relative amounts of 3 polypeptides. The addition of 84 mM malate to resting cells increased the relative amounts of 3 of the ethanol-induced polypeptides, suggesting that the induction of these polypeptides represents a generalized response to the provision of a utilizable carbon source, a nutritional shift up, rather than a specific response to ethanol addition. Exposure of dark-grown resting Euglena to light increases the relative amounts of 79 polypeptides encoded by the nuclear as well as the chloroplast genome and decreases the relative amounts of 72 polypeptides. Ethanol but not malate specifically inhibited all of the light-dependent changes in polypeptide levels, indicating that chloroplast development in Euglena is a catabolite-sensitive process.

Regulation of synthesis of citrate synthase in regreening Euglena gracilis

Exposure of dark-grown Euglena to white or red light, but not blue light, produced a twofold increase in the specific activity of citrate synthase. A 400-fold purification of mitochondrial citrate synthase (subunit Mr = 44000) was achieved from cells of Euglena gracilis by affinity chromatography on ATP-activated agarose. Antisera, raised against the homogeneously pure enzyme, were used to demonstrate that the increase in citrate synthase activity on exposure of dark-grown cells to light resulted from an increase in citrate synthase protein. Anti-(citrate synthase) was used to detect precursor citrate synthase resulting from the translation of total polyadenylated RNA from Euglena in a cell-free rabbit reticulocyte lysate system. Citrate synthase mRNA was found to be present in cells at all stages of regreening. However, extraction and translation of polyadenylated RNA from free polysomes isolated from darkgrown and regreening cells demonstrated that appreciable translation of citrate synthase mRNA was only occurring in regreening cells.

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.