Regulation of galactose oxidase synthesis and secretion in Dactylium dendroides: effects of pH and culture density

Expression, purification, and characterization of galactose oxidase of Fusarium sambucinum in E. coli

A gene encoding a galactose oxidase (GalOx) was isolated from Fusarium sambucinum cultures and overexpressed in Escherichia coli yielding 4.4mg enzyme per L of growth culture with a specific activity of 159Umg(-1). By adding a C-terminal His-tag the enzyme could be easily purified with a single affinity chromatography step with high recovery rate (90%). The enzyme showed a single band on SDS-PAGE with an apparent molecular mass of 68.5kDa. The pH optimum for the oxidation of galactose was in the range of pH 6-7.5. Optimum temperature for the enzyme activity was 35°C, with a half-life of 11.2min, 5.3min, and 2.7min for incubation at 40°C, 50°C, and 60°C, respectively. From all tested substrates, the highest relative activity was found for 1-methyl-β-galactopyranoside (226Umg(-1)) and the highest catalytic efficiency (kcat/Km) for melibiose (2700mM(-1)s(-1)). The enzyme was highly specific for molecular oxygen as an electron acceptor, and showed no appreciable activity with a range of alternative acceptors investigated. Different chemicals were tested for their effect on GalOx activity. The activity was significantly reduced by EDTA, NaN3, and KCN.

A new PCR approach for the identification of Fusarium graminearum

The main objective of this work was to develop a PCR protocol for the identification of Fusarium graminearum, based on a pair of primers targeted to a segment of the 3´coding region of the gaoA gene that codes for the enzyme galactose oxidase (GO). This region has low homology with the same region of GO genes from other fungi. Genomic DNA from 17 strains of Fusarium spp. isolated from diseased cereals, from several other Fusarium species, and from other fungi genera was analyzed in a PCR assay using this primer set. The 17 strains of Fusarium spp. were also analyzed for the GO enzyme production in submerse fermentation in a new formulated liquid medium. All strains that were morphologically and molecularly identified as F. graminearum were able to secrete the enzyme and had a positive result in the used PCR protocol. No DNA fragment was amplified using genomic DNA from other Fusarium species and species of other fungi genera. The results suggest that the proposed PCR protocol is specific and can be considered as a new molecular tool for the identification of F. graminearum. In addition, the new formulated medium is a cheap alternative for screening for GO screening production by F. graminearum.

Production, purification, and characterization of a novel galactose oxidase fromFusarium acuminatum

Extra-cellular production of a novel galactose oxidase from Fusarium acuminatum using submerged fermentation was studied. Glucose (1.0% w/v) was used as the sole carbon source. Maximum galactose oxidase production (approximately 4.0 U/ml) was obtained when fermentation was carried out at 25 degrees C, with orbital shaking (100 rpm) and an initial medium of pH 7.0, for 96 h, using a 2% (v/v) inoculum made from a homogenized four-day-old liquid culture, in the presence of copper, manganese, and magnesium. The enzyme was purified by one-step affinity chromatography, with a recovery of 42% of the initial activity. The purified enzyme ran as a single band of 66 kDa in SDS-PAGE. Optimal pH and temperature for the enzyme activity were 8.0 and 30 degrees C, respectively. The enzyme was thermoinactivated at temperatures above 60 degrees C. The purified enzyme was active toward various substrates, including galactose, dihydroxyacetone, guar gum, lactose, melibiose, methyl-galactopyranoside, and raffinose. SDS was an inhibitor but EDTA, Tween 80, NH(4)(+), Na(+), Mg(2+), K(+), and glycerol were not. The Michaelis-Menten constant (K(m)) for galactose was estimated to be 16.2 mM, while maximal velocity (V(max)) was 0.27 micromol of H(2)O(2) . ml(-1) . min(-1).

Secretion of phospholipase C by Pseudomonas aeruginosa

The conditions necessary for the secretion of phospholipase C (phosphatidylcholine cholinephosphohydrolase) by Pseudomonas aeruginosa were studied. Enzyme secretion by washed cell suspensions required a carbon source and ammonium, potassium, and calcium ions. The calcium requirement could be substituted by magnesium and strontium but not by copper, manganese, cobalt, or zinc. During growth in liquid medium, cells secreted phospholipase C during late logarithmic and early stationary phases. Secretion was repressed by the addition of inorganic phosphate but not by organic phosphates, glucose, or sodium succinate. Studies with tetracycline indicated that de novo protein synthesis was necessary for the secretion of phospholipase C and that the exoenzyme was not released from a preformed periplasmic pool. Similarly, extraction of actively secreting cells with 0.2 M MgCl2 at pH 8.4 solubilized large quantities of the periplasmic enzyme alkaline phosphatase but insignificant amounts of phospholipase C. Bacteria continued to secrete enzyme for nearly 45 min after the addition of inorganic phosphate or rifampin.

Biosynthesis and cellular distribution of the two superoxide dismutases of Dactylium dendroides

The synthesis and subcellular localization of the two superoxide dismutases of Dactylium dendroides were studied in relation to changes in copper and manganese availability. Cultures grew normally at all medium copper concentrations used (10 nM to 1 mM). In the presence of high (10 muM) copper, manganese was poorly absorbed in comparison to the other metals in the medium. However, cells grown at 10 nM copper exhibited a 3.5-fold increase in manganese content, while the concentration of the other metals remained constant. Cultures grown at 10 nM copper or more had 80% Cu/Zn enzyme and 20% mangani enzyme; the former was entirely in the cytosol, and the latter was mitochondrial. Removal of copper from the medium resulted in decreased Cu/Zn superoxide dismutase synthesis with a concomitant increase in the mangani enzyme such that total cellular superoxide dismutase activity remained constant. The mangani enzyme in excess of the 20% was present in the non-mitochondrial fraction. The mitochondria, therefore, show no variability with respect to superoxide dismutase content, whereas the soluble fraction varies from 100 to 13% Cu/Zn superoxide dismutase. Copper-starved cells that were synthesizing predominantly mangani superoxide dismutase could be switched over to mostly Cu/Zn superoxide dismutase synthesis by supplementing the medium with copper during growth. Immunoprecipitation experiments suggest that the decrease in Cu/Zn activity at low copper concentration is a result of decreased synthesis of that protein rather than the production of an inactive apoprotein.

The utilization of copper and its role in the biosynthesis of copper-containing proteins in the fungus, Dactylium dendroides

Aspects of the utilization of copper by the fungus, Dactylium dendroides, have been studied. The organism grows normally at copper levels below 10 nM. Cells grown in medium containing 30 nM copper or less concentrate exogenous metal at all levels of added copper; copper uptake is essentially complete within 15 min and is not inhibited by cycloheximide, dinitrophenol or cyanide. These results indicate that copper absorption is not an energy-dependent process. The relationship between fungal copper status and the activities of three copper-containing enzymes, galactose oxidase, and extracellular enzyme, the cytosolic, Cu/Zn superoxide dismutase and cytochrome oxidase, has also been established. The synthesis of galactose oxidase protein (holoenzyme plus apo-enzyme) is independent of copper concentration. Cells grown in copper-free medium (less than 10 nM copper) excrete normal amounts of galactose oxidase as an apoprotein. At medium copper levels below 5 micrometer, new cultures contain enough total copper to enable the limited number of cells to attain sufficient intracellular copper to support hologalactose oxidase production. As a result of cell division, however, the amount of copper available per cell drops to a threshold of approx. 10 ng/mg below which point only apogalactose oxidase is secreted. Above 5 micrometer medium copper, holoenzyme secretion is maintained throughout cell growth. The levels of the Cu/Zn superoxide dismutase respond differently in that the protein itself apparently is synthesized in only limited amounts in copper-depleted cells. Total cellular superoxide dismutase activity is maintained under such conditions by an increase in activity associated with the mitochondrial, CN(-)-insensitive, manganese form of this enzyme. Cells grown at 10 micrometer copper show 83% of their superoxide dismutase activity to be contributed by the Cu/Zn form compared to a 17% contribution to the total activity in cells grown at 30 nM copper, indicating that the biosynthesis of the Cu/Zn and Mn-containing enzymes is coordinated. The data show that the level of copper modulates the synthesis of the cytosolic superoxide dismutase. In contrast, the cytochrome oxidase activity of D. dendroides is independent of cellular copper levels obtainable. Thus, the data also suggest that these three enzymes utilize different cellular copper pools. As cells are depleted of copper by cell division, the available copper is used to maintain Cu/Zn superoxide dismutase and cytochrome oxidase activity; at very low levels of copper, only the latter activity is maintained. The induction of the manganisuperoxide dismutase in copper-depleted cells should have practical value in the isolation of this protein.