Malate synthase messenger RNA in relation to enzyme derepression in Euglena gracilis

Characterization and molecular properties of 2-oxoglutarate decarboxylase from Euglena gracilis

2-Oxoglutarate decarboxylase was purified to homogeneity, as judged by polyacrylamide gel electrophoresis. It had a molecular weight of 250,000 and consisted of four identical subunits of molecular weight 62,000. The enzyme was specific for 2-oxoglutarate, but not for other 2-oxo acids such as pyruvate and oxalacetate. Thiamin pyrophosphate and MgCl2 were required for maximum activity. The Km values of the enzyme for 2-oxoglutarate, thiamin pyrophosphate, and MgCl2 were 330, 56, and 93 microM, respectively. 2-Mercaptoethanol and NADP+ augmented significantly the enzyme activity. The amino acid composition and amino acid sequence of the amino-terminal region of 2-oxoglutarate decarboxylase were determined. On ouchterlony double-immunodiffusion gels, the anti-2-oxoglutarate decarboxylase antibody gave sharp precipitin lines against the mitochondrial fraction of E. gracilis and the purified 2-oxoglutarate decarboxylase, but not against pyruvate decarboxylase from Saccharomyces cerevisiae. On Immunoblots of the crude extract of Euglena, the antibody recognized two polypeptides whose molecular weights were 62,000 and 65,000, respectively. The polypeptide with the molecular weight of 62,000 was found only in mitochondrial fractions. In vitro translation of Euglena polyadenylated RNA in a cell-free rabbit reticulocyte lysate system explained the formation of a single polypeptide with a molecular weight of 65,000, suggesting that a putative precursor of 2-oxoglutarate decarboxylase which is about 3000 larger than the subunit of the mature enzyme is synthesized in Euglena cells.

Glutamate dehydrogenase (NADP-dependent) mRNA in relation to enzyme synthesis in Euglena gracilis. Evidence for post-transcriptional control

Cells of Euglena gracilis Klebs strain z Pringsheim had high NADP-dependent glutamate dehydrogenase activity when grown on glutamate as nitrogen source but activity was completely repressed in cells grown on ammonium (NH4+). A 120-fold purification of NADPH-glutamate dehydrogenase (subunit Mr = 45 000) was achieved from glutamate-grown cells by affinity chromatography on blue Sepharose CL-6B. Antisera raised against the homogeneously pure protein were used to demonstrate that increase in NADPH-glutamate dehydrogenase activity on transfer from NH4+ to glutamate medium resulted from an increase in the amount of protein. Glutamate NH4+-grown cells were labelled with L-[35S]methionine and anti-(NADPH-glutamate dehydrogenase) used to immunoprecipitate the dehydrogenase from cell extracts. NADPH-glutamate dehydrogenase protein was detected in glutamate-grown but not NH4+-grown cells. Anti-(NADPH-glutamate dehydrogenase) was used to detect NADPH-glutamate dehydrogenase resulting from the translation of total polyadenylated RNA from Euglena in a cell-free rabbit reticulocyte lysate system. NADPH-glutamate dehydrogenase mRNA was present in glutamate NH4+-grown cells, there being no apparent difference in mRNA abundance between cells showing a tenfold difference in NADPH-glutamate dehydrogenase specific activity. These results indicate that the synthesis of this dehydrogenase is regulated primarily at the post-transcriptional level.

Citrate-synthase mRNA in relation to enzyme synthesis in division-synchronized Euglena cultures

The regulation of citrate-synthase (EC 4.1.3.7) synthesis in division-synchronized cultures of Euglena gracilis Klebs strain z was investigated. Citrate-synthase activity increased approximately two fold at the end of the light phase and in early dark phase in phototrophic cells synchronized by a 14 h light-10 h dark regime. Anti-(citrate synthase) was used to demonstrate that this increase in activity resulted from an increase in citrate-synthase protein. The amount of polyadenylated RNA per aliquot of culture increased midway through the light phase (before DNA replication) and had doubled by the end of this phase. Anti-(citrate synthase) was used to detect precursor citrate synthase in translations of total polyadenylated RNA in rabbit reticulocyte lysates. Citratesynthase mRNA was found to be present in cells at each stage of a division cycle, so that a stagespecific production of this mRNA to coincide with an increase in enzyme protein is unlikely. It is suggested that a post-transcriptional control operates in the regulation of citrate-synthase synthesis.

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.