Identification and oligosaccharide structure analysis of rhodopsin glycoforms containing galactose and sialic acid

The N-linked oligosaccharides of frog (Rana pipiens) rhodopsinwere analysed by sequential exoglycosidase digestion and gelfiltration chromatography, following reductive tritiation. Inaddition, selected tryptic glycopeptides obtained from frogretinal rod outer segment membranes were examined by electrospraymass spectrometry (ES-MS), fast atom bombardment mass spectrometry(FAB-MS), amino acid sequence and composition analysis, andcarbohydrate composition analysis. The amino acid sequence datademonstrated that the glycopeptides were derived from rhodopsinand confirmed the presence of twoN-glycosylation sites, at residuesAsn² and Asn¹⁵. The predominant glycan (60% of total) had thestructure GlcNAcß1–2Man1–3(Man1–6)Manß1–4GlcNAcß1–4GlcNAc-(Asn),with the remaining structures containing 1–3 additionalhexose residues, as reported previously for bovine rhodopsin.Unlike bovine rhodopsin, however, a sizable fraction of thetotal giycans of frog rhodopsin also contained sialic acid (NeuAc),with the sialylated oligosaccharides being present exclusivelyat the Asn² site. FAB-MS analysis of oligosaccharides releasedfrom the Asn² site gave, among other signals, an abundant quasimolecularion corresponding to a glycan of composition NeuAc₁Hex₆HexNAc₃(where Hex is hexose and HexNAc is N-acetylhexosamine), consistentwith a hybrid structure. The potential biological implicationsof these results are discussed in the context of rod outer segmentmembrane renewal. glycoforms oligosaccharide structure rhodopsin     

The Osmotic Pressure of Concentrated Solutions of Polyethylene Glycol 6000, and its Variation with Temperature

The vapour pressures of aqueous solutions of polyethylene glycol6000 have been measured (by equilibration with sucrose solutions)up to the saturation point at 25 °C (1.45 g g^–1 water).The reduced-osmotic-pressure (/c), when plotted versus concentration(c), rapidly and linearly increased up to a concentration ofabout 0.8 g g^–1 (crossing the similar plot for sucrose).Above this concentration, the reduced-osmotic-pressure rosemore slowly, but still more rapidly than sucrose. The maximumosmotic pressure achieved at saturation was nearly 18 MPa. Usingthe virial equation: /c= RT/M + RTA₂c, the calculated secondvirial coefficient (A₂) for the linear part is 4.5 x 10^–3mol g^–1, a value slightly greater than most literaturevalues at 25 °C. Data are cited showing that A₂ varies linearlyfrom 5–6 x 10^x3 at 0 °C, to zero at 80–90 °C     

Homonojirimycin and N-methyl-homonojirimycin inhibit N-Iinked oligosaccharide processing

Homonojirimycin (HNJ) and N-methylhomonojirimycin (MHNJ) weretested as inhibitors of the purified glycopro-tein processingenzymes, glucosidase I and glucosidase II. MHNJ was a reasonablygood inhibitor of glucosidase I (K₁ = 1 x 10^–6 M) andwas about three times as effective on this enzyme as was HNJ.On the other hand, HNJ inhibited glucosidase II with a K₁ ofabout 1 x 10^x6 M, whereas MHNJ was three times less effective(K₁ = 3 x 10^–5 M). However, the butyl derivative of HNJhad very low activity toward these two processing glucosidases.HNJ and its methyl derivative were also tested in vivo usinginfluenza virus-infected MDCK cells, and measuring the inhibitionof N-linked oligosaccharide processing of the viral envelopeglycoproteins. With 100 µg/ml of MHNJ in the medium, essentiallyall of the N-linked oligosaccharide chains of the virus wereof the "high-mannose" type with the major structure being characterizedas Glc₃Man₉(GlcNAc)₂. Similar results were obtained with HNJalthough this compound was less effective in vivo as well asin vitro. These results are in keeping with these inhibitorsbeing effective at the glucosidase I step. Both inhibitors werealso tested in MDCK cell cultures to determine whether theyaffected the in vivo synthesis of proteins, or of lipid-linkedsaccharides. In contrast to deoxynojirimycin, which has beenreported to inhibit the formation of lipid-linked saccharides,no effects were seen on either the incorporation of mannoseinto lipid-linked saccharides or the incorporation of leucineinto protein. glucosidase lipid influenza virus oligosaccharide     

Nitrate transport in intact wheat roots:II. Long-term effects of NO3- concentration in the nutrient solution on NO3- unidirectional fluxes and distribution within the tissues5

We have examined the long-term effects of NO₃^– concentrationson NO₃^– (¹⁵NO₃^–) fluxes and cellular pool sizesin roots of intact 30-d-old wheat (Triticum aestivum cv. Courtot)grown hydroponically. Compartmental analysis was performed understeady-state conditions at five different levels of NO₃^–concentration (from 0.1 up to 5 mol m^–3 taking into accountmetabolism and secretion into the xylem (Devienne et al., 1994).Nitrate and reduced nitrogen levels in the tissues were largelyindependent of external NO₃^– concentration although below1.5 mol m^–3 NO₃^–; concentration limited plant growth.In the chamber, marked diurnal variations in net uptake occurredand, in the light, higher NO₃^– concentrations yieldedhigher NO₃^– uptake rates. After transfer of the plantsto the laboratory, the increase in net uptake linked to elevationof NO₃^–; concentrations was even larger (from 0.1 to 8.8µmolh^–1 g^–1 FW) as a result of a marked increase (x10–11) in the unidirectional influx at the plasmalemmawhile NO₃^– efflux was less enhanced (x 4–5). Underthese conditions, influx into the vacuole was also higher (x2–4) while efflux from the vacuole was little affected(x 1–3). NO₃^– concentrations within the cell compartmentswere estimated under the clas sical assumptions. The vacuolarconcentration was a little modified by NO₃^– availabilitywhereas that in the cytosol increased from about 10 mol m^–3to about 20 mol m^–3 indicating that (1) the absolute valuefor the cytosol was high and (2) it displayed only a small increasedespite very large changes in NO₃^– fluxes. NO₃^–distribution within the cells did not seem to involve an activeaccumulation of NO₃^– in the vacuole. Key words: Wheat, ion transport, nitrate, ¹⁵N, compartmentation     

Effect of complete protein 4.1R deficiency on ion transport properties of murine erythrocytes

Moderate hemolytic anemia, abnormal erythrocyte morphology (spherocytosis), and decreased membrane stability are observed in mice with complete deficiency of all erythroid protein 4.1 protein isoforms (4.1^–/–; Shi TS et al. J Clin Invest 103: 331, 1999). We have examined the effects of erythroid protein 4.1 (4.1R) deficiency on erythrocyte cation transport and volume regulation. 4.1^–/– mice exhibited erythrocyte dehydration that was associated with reduced cellular K and increased Na content. Increased Na permeability was observed in these mice, mostly mediated by Na/H exchange with normal Na-K pump and Na-K-2Cl cotransport activities. The Na/H exchange of 4.1^–/– erythrocytes was markedly activated by exposure to hypertonic conditions (18.2 ± 3.2 in 4.1^–/– vs. 9.8 ± 1.3 mmol/10¹³ cell x h in control mice), with an abnormal dependence on osmolality (EC₅₀ = 417 ± 42 in 4.1^–/– vs. 460 ± 35 mosmol/kgH₂O in control mice), suggestive of an upregulated functional state. While the affinity for internal protons was not altered (K_0.5 = 489.7 ± 0.7 vs. 537.0 ± 0.56 nM in control mice), the V_max of the H-induced Na/H exchange activity was markedly elevated in 4.1^–/– erythrocytes (V_max 91.47 ± 7.2 compared with 46.52 ± 5.4 mmol/10¹³ cell x h in control mice). Na/H exchange activation by okadaic acid was absent in 4.1^–/– erythrocytes. Altogether, these results suggest that erythroid protein 4.1 plays a major role in volume regulation and physiologically downregulates Na/H exchange in mouse erythrocytes. Upregulation of the Na/H exchange is an important contributor to the elevated cell Na content of 4.1^–/– erythrocytes. spherocytosis; cell Na; Na/H exchange     

The Partitioning of Nitrate Assimilation Between Root and Shoot of a Range of Temperate Cereals and Pasture Grasses

Nitrate reductase activity (NRA, in vivo assay) and nitrate(NO^-₃) content of root and shoot and NO^-₃ and reduced nitrogencontent of xylem sap were measured in five temperate cerealssupplied with a range of NO^-₃ concentrations (0·1–20mol m^–3) and three temperate pasture grasses suppliedwith 0·5 or 5 0 mol m^–3 NO^-₃ For one cereal (Hordeumvulgare L ), in vitro NRA was also determined The effect ofexternal NO^-₃ concentration on the partitioning of NO^-₃ assimilationbetween root and shoot was assessed All measurements indicatedthat the root was the major site of NO3 assimilation in Avenasatwa L, Hordeum vulgare L, Secale cereale L, Tnticum aestivumL and x Triticosecale Wittm supplied with 0·1 to 1·0mol m^–3 NO^-₃ and that for all cereals, shoot assimilationincreased in importance as applied NO^-₃ concentration increasedfrom 1.0 to 20 mol m^–3 At 5.0–20 mol m^–3 NO3,the data indicated that the shoot played an important if notmajor role in NO^-₃ assimilation in all cereals studied Measurementson Lolium multiflorum Lam and L perenne L indicated that theroot was the main site of NO^-₃ assimilation at 0.5 mol m^–3NO^-₃ but shoot assimilation was predominant at 5.0 mol m^–3NO^-₃ Both NRA distribution data and xylem sap analysis indicatedthat shoot assimilation was predominant in Dactylis glomerataL supplied with 0.5 or 5.0 mol m^–3 NO^-₃ Avena sativa L., oats, Hordeum vulgare L., barley, Secale cereale L., rye, x Triticosecale Wittm., triticale, Triticum aestivum L., wheat, Dactylis glomerata L., cocksfoot, Lolium multiflorum Lam., Italian ryegrass, Lolium perenne L., perennial ryegrass, nitrate, nitrate assimilation, nitrate reductase activity, xylem sap     

Nitrate Effects on Growth of the First Four Main Stem Leaves of a Range of Temperate Cereals and Pasture Grasses

The effects of different applied NO₃^– concentrations onextension growth and final length and area of leaves 1–4of five cereals and six pasture grasses of temperate originwere examined. Increased applied NO₃^– in the range 0.1–0.5.0mol m^–3 caused decreased duration of growth but increasedgrowth rate and final length of leaves 2–4 of the cerealsAvena saliva, Hordeum vulgare, Secale cereale, x Triticosecaleand Triticum aestivum. For all cereals, increased NO₃^–resulted in increased area of leaves 1–4. Pasture grasseswere supplied either 0.5 or 50 mol m^–3 NO₃^–. Increasedapplied NO₃^– (0.5–5.0 mol m^–3) resulted indecreased duration of growth and increased growth rate and finalarea of leaves 1–4 of Bromus wiltdenowii, leaves 2–4ofFestuca arundinaceae and leaves 3 and 4 of Lolium muitiflorum.In addition, length of leaves 3 and 4 of B. witidenowii increasedwith increased NO₃^–. Increased NO₃^– resulted inincreased area of leaves 2–4 of Dactylis gtomerata andLolium perenne and leaves 3 and 4 of Phalaris aquaiica but hadno effect on extension growth of all three species. Avena sativa L, oat, Hordeum vulgare L, barley, Secale cereale L, rye, x Triticosecale Wittm, triticale, Triticum aestivum L, wheat, Bromus willdenowii Kunth, prairie grass, Dactylis gtomerata L, cocksfoot, Festuca arundinaceae Shreb, tall fescue, Lolium multijlorum Lam, Italian ryegrass, Lolium perenne L, perennial ryegrass, Phalaris aquatica L, nitrate, leaf extension, leaf expansion     

Measuring the Canopy Net Photosynthesis of Glasshouse Crops

A null balance method is described for measuring net photosynthesisof mature canopies of cucumber and other protected crops overperiods of 10 min in a single-span glasshouse (c. 9m x 18m inarea). Accuracy of control of the CO₂ concentration in the greenhouseatmosphere is within ±10 vpm of the normal ambient level(c. 350 vpm). The amounts of CO₂ used in canopy net photosynthesisare measured with linear mass flowmeters accurate to within±0.80g. The total errors incurred in measuring canopynet photosynthesis at an ambient CO₂ level are estimated tobe of the order of ± 1·2% in bright light (350W m^–2, PAR)and ±3·6% in dull light (100W m^–2, PAR). Measurements of the rates of net photosynthesis of a maturecanopy of a cucumber crop were made at near-ambient CO₂ concentrationsover a range (0–350 W m^–2) of natural light fluxdensities. A model of light absorption and photosynthesis applicableto row crops was used to obtain a net photosynthesis versuslight response curve for the cucumber crop. At a light fluxdensity of 350 W m^–2 the fitted value of canopy net photosynthesiswas 2.65 mg CO₂ m^–2s^–1 (equivalent to over 95 kgCO₂ ha^–1h^–1). The results are discussed in relationto the need for CO₂ supplements to avoid depletion in both ventilatedand unventilated glasshouses during late spring and summer. Key words: Glasshouse crops, cucumber, measurement, canopy photosynthesis, light, CO₂     

Skeletal keratan sulphate structural analysis using keratanase II digestion followed by high-performance anion-exchange chromatography

A semi-quantitative fingerprinting method has been developedfor the structural analysis of skeletal keratan sulphates. Thisinvolves the digestion of the parent keratan sulphate chainswith the enzyme keratanase II (Bacillus sp.), followed by reductionof the resulting oligosaccharides with sodium borohydride andchromatography on a Dionex AS4A-SC column. This column has beencalibrated for the elution positions of 26 previously characterizedoligosaccharides (Brown et al., Biochemistry, 33, 4836–4846,1994; Brown et al., Eur. J. Biochem., 224, 281–308, 1994).The technique permits sample analysis with pulsed electrochemicaldetection (sensitive to     

Nitrate Effects on Growth of the First Four Main Stem Leaves of a Range of Temperate Cereals and Pasture Grasses

The effects of different applied NO₃^– concentrations onextension growth and final length and area of leaves 1–4of five cereals and six pasture grasses of temperate originwere examined. Increased applied NO₃^– in the range 0.1–50mol m^–3; caused decreased duration of growth but increasedgrowth rate and final length of leaves 2–4 of the cerealsAvena saliva, Hordeum vulgare, Secale cereale x Triticosecaleand Triticum aestivum. For all cereals, increased NO₃^–resulted in increased area of leaves 1-4. Pasture grasses weresupplied either 0.5 or 50 mol m^–3; NO₃^–. Increasedapplied NO₃^– (0.5–50 mol m^–3) resulted indecreased duration of growth and increased growth rate and finalarea of leaves 1–4 of Bromus willdenowii leaves 2–4of Festuca arundinaceae and leaves 3 and 4 of Lolium multiflorum.In addition, length of leaves 3 and 4 of B. willdenowii increasedwith increased NO₃. Increased NO₃ resulted in increased areaof leaves 2–4 of Daciylis glomerata and Lolium perenneand leaves 3 and 4 of Phalaris aquatica but had no effect onextension growth of all three species. Avena saliva L., oat, Hordeum vulgare L., barley, Secale cereaie L., rye, x Triticosecale Wittm, triticale, Triticum aestivum L., wheat, Bromus willdenowii Kunth, prairie grass, Dactylis glomerata L., cocksfoot, Festuca arundinaceae Shreb, tall fescue, Lolium multiflorum Lam, Italian ryegrass, Lolium perenne L, perennial ryegrass, Phalaris aquatica L, nitrate,, leaf extension, leaf expansion     

Properties of the Cytochrome c Oxidase Activity of Cytochrome b561 from Photoanaerobically Grown Rhodopseudomonas sphaeroides

Cytochrome b₅₆₁ from Rhodopseudomonas sphaeroides had cytochromec (c2) oxidase activity and a pH optimum at 6.0 for this activity.The activity was affected by the ionic strength of the reactionmixture. The apparent K_m and maximal velocity (V_max) valuesin the absence of addea salts were 14 µM and 120 nmoloxidized per min per mg protein for horse heart cytochrome c.Reduced horse heart cytochrome c was reoxidized in first-orderkinetics by this cytochrome b₅₆₁. The specific activity was0.7 s^–1 per mg protein at 20°C at the concentrationof 30 µMM cytochrome c. Activity was inhibited by KCN and NaN₃, but not by antimycin.The addition of a low concentration of KCN to the cytochromeb₅₆₁ produced a change in the absorption spectrum, evidencethat KCN interacts with the heme moiety of cytochrome b₅₆₁.Results of this and preceeding studies show that the cytochromeoxidase (cytochrome "o") described earlier (Sasaki et al. 1970)is cytochrome b₅₆₁. (Received May 16, 1983; Accepted September 8, 1983)     

L-Leucine Transport in Isolated Protoplasts of Vinca Suspension Culture: Characterization of Uptake

The uptake of L-leucine into Vinca protoplasts was studied undervarious conditions. The uptake was highly pH-dependent, withthe optimal pH between 3.0 and 4.0. The uptake was also energydependent, since azide, 2,4-dinitrophenol (DNP), carbonyl cyanidem-chlorophenyl hydrazone (CCCP), and iodoacetate inhibited theuptake. Oligomycin, N,N'-dicycIohexyI carbodiimide (DCCD) andvanadate, but not ouabain, inhibited the uptake, suggestingthat ATPase for H⁺ electrogenic extrusion was necessary to theuptake of L-leucine. The uptake showed stereospecificity, butwas partially inhibited by other L-amino acids. A kinetic studyof the uptake showed that the uptake was multiphasic with threesaturable phases and one unsaturable phase which occurred atconcentrations of L-leucine over 1 mM. The K_m values of thethree affinity sites were 1.4 x 10^–3 M, 1.3 x 10^–4M, 4.3 x 10^–5 M; the maximum velocity values were 3.3x 10^–8, 4.5 x 10^–9, 1.8 x 10_–9 mol/10 min/4x 10⁶ cells. (Received April 18, 1981; Accepted August 25, 1981)     

Participation of Inorganic Orthophosphate in Regulation of the Ribulose-1,5-bisphosphate Carboxylase Activity in Response to Changes in the Photosynthetic Source-Sink Balance

Sink-limited conditions, defined as treatment with continuousillumination, cause a reduction in the rate of photosyntheticfixation of CO₂ in single-rooted leaves of soybean (Glycinemax. Merr. cv. Turunoko). We suggested previously that thisreduction is due to a deactivation of ribulose-1,5-bisphosphatecarboxylase (RuBPcase, E.C. 4.1.1.39 [EC] ) that is caused by a decreasein the level of P_i in the leaves [Sawada et al. (1989) PlantCell Physiol. 30: 691, Sawada et al. (1990) Plant Cell Physiol.31: 697]. In the present study, the mechanism of regulationof RuBPcase activity by P_i was examined. The activity of RuBPcasein the sink-limited leaves, exposed for 6 or 7 d to continuousillumination to alter the source/sink balance, was enhancedwith increasing concentrations of P_i, in a CO₂-free preincubationmedium in the presence of 5 mM MgCl₂ The maximum value [6.3µmole CO₂ (mg Chl)^–1 min^–1] was obtained atapproximately 5 mM P_i after a 5 min incubation, being 3 timesof the activity without the preincubation. The activity of acrude preparation of RuBPcase that had been deactivated by removalof CO₂ and Mg²⁺ ions by the gel filtration was 5.2–9.3nmole CO₂ (mg protein)^–1 min^–1 and was also enhancedby P_i plus Mg²⁺ ions. The maximum value [147–151 nmoleCO₂ (mg protein)^–1 min^–1] was attained at 5 mM P_iafter a 5 min incubation. The cycle of activation and inactivationof deactivated crude RuBPcase was perfectly reversible by additionof P_i to the enzyme and removal of P_i from the enzyme. Levelsof free P_i and of esterified phosphate in the sink-limited leaveswere 69% and 31% of the total phosphate, respectively. By contrast,in the control leaves, these values were 87% and 13%, respectively.These results support our previously stated hypothesis and indicatean important role for free P_i in the regulation of RuBPcaseactivity, in particular in sink-limited plants. (Received February 21, 1992; Accepted July 23, 1992)     

The NADP$-specific isocitrate dehydrogenase of Rhodospirillum rubrum

The NADP^$-specific isocitrate dehydrogenase was partially purifiedfrom photosynthetically-grown Rhodospirillum rubrum. The pHoptimum is between 7.5 and 9.0 in phosphate buffer. The apparentKm is 3.1x10^–5 M for isocitrate, 5.1x10^–5 M forNADP^$, 1.7x10^–5 M for manganese, 1.5x10^–4 M formagnesium, and 3.5x10^–3 M for inorganic orthophosphate.Arsenate exerts a slight inhibition. The Q₁₀ between 17.5°Cand 40°C is 1.62, and the energy of activation at 25°Cis 9.74 Kcal/mole. Glyoxylate and oxalacetate cause concertedinhibition of the enzyme activity. Various nucleotides inhibitthe activity. The kinetics of inhibition by ATP was found tobe mixed type with respect to NADP^$ and isocitrate, the K_i valuesbeing 1.17x10^–3 M and 1.10x10^–3 M respectively.The inhibition between ATP and orthophosphate is competitivewith a K_i of 10^–4M. Thiol binding reagents are inhibitory;this inhibition is reversed by cysteine or reduced glutathione. (Received October 1, 1971; )     

The Influence of the Concentration of Gaseous Ammonia on its Uptake by the Leaves of Italian Ryegrass, with and without an Adequate Supply of Nitrogen to the Roots

Whitehead, D. C. and Lockyer, D. R. 1986. The influence of theconcentration of gaseous ammonia on its uptake by the leavesof Italian ryegrass, with and without an adequate supply ofnitrogen to the roots.—J. exp. Bot. 38: 818–827. Plants of Italian ryegrass (Lolium multiflorum Lam.) were grownin pots of soil with two rates of ¹⁵N-labclled nitrate, oneproviding adequate, and the other less than adequate, N formaximum growth. After 25 d in a controlled environment cabinet,the plants were transferred to chambers and exposed for 33 dto NH₃in the air at one of nine concentrations ranging from14 to 709 µg NH₃ m^–3. Increasing the concentrationof NH₃ in the air increased the dry weight of the shoots ofplants grown at the lower but not the higher rate of nitrate.The content of total N in the plant shoots (% dry weight) waslinearly related to NH₃ concentration; at 709 µg NH₃ andin both sets of plants it was more than double the content at14 µg NH₃ m^–3. Calculations, based on ¹⁵N enrichment,indicated that the amount of N taken up from the NH₃ per unitleaf area increased linearly with increasing concentration ofNH₃ in the air uptake (µg dm^–2 h^–1) = 0.1009xat the lower rate of nitrate and 0-0829x at the higher rateof nitrate, where x is the concentration of NH₃ in the air expressedas µg NH₃m^–3. The proportion of the total plant N that was derived from theNH₃ ranged from 4?0% at a concentration of 14 µg NH₃ m^–3with the higher rate of nitrate addition to 77?5% at a concentrationof 709 µg m^–3 with the lower rate of nitrate addition.The proportions of the total N in the water-insoluble proteinof the leaf tissue that were derived from nitrate and gaseousNH₃ were similar to the proportions in the whole leaf material. Key words: Ammonia, nitrogen, leaf sorption, Lolium multiflorum     

Identification of a GDP-L-fucose:polypeptide fucosyltransferase and enzymatic addition of O-linked fucose to EGF domains

An assay for GDP-fucose:polypeptide fucosyltransferase has beenestablished. The enzyme catalyzes the reaction that attachesfucose through an O-glycosidic linkage to a conserved serineor threonine residue in EGF domains. The assay uses recombinanthuman factor VII EGF-1 domain as acceptor substrate and GDP-fucoseas donor substrate. Synthetic peptides with sequences takenfrom five proteins previously shown to contain O-linked fucose(Harris and Spellman, 1993; Glycobiology 3, 219–224) didnot serve as efficient acceptor substrates. These syntheticpeptides did not comprise complete EGF domains and did not containall six cysteine residues that define the EGF structure. Therefore,the enzyme appears to require more than just a consensus primarysequence and likely requires that the EGF domain disulfide bondsbe properly formed. The enzymatic reaction showed linear dependencyof its activity on time, amount of enzyme, and substrates. Althoughthe enzyme did not exhibit an absolute requirement for Mn²⁺enzymatic activity did increase ten fold in the presence of50 mM MnCl₂. The in vitro glycosylation reaction resulted incomplete conversion of the acceptor substrate to glycosylatedproduct, and characterization of the purified product by electrospraymass spectrometry revealed that one fucose was added onto thepolypeptide. Most of the enzymatic activity was found to bein the soluble fraction of CHO cell homogenates. However, whenenzyme was prepared from rat liver in the presence of proteaseinhibitors, 37% of the activity was recovered by Triton X-100extraction of the membrane particles after extensive aqueouswashes. The result suggests that the enzyme is probably a membraneprotein and, by analogy with other glycosyl transferases, probablyhas a ‘stem’ region that is very susceptible toproteolysis. fucosyltransferase O-linked fucose EGF domain glycosylation     

Strategies d'adaptation photosynthetique chez des diatomees des I'Ocean Antarctique: variations du nombre et de la talile des unites photosynthetiques

Les diatomes Nitzschia turgiduloides et Chaetoceros deflandrei,rcoltes en fvrier–Mars 1980 dans le secteur indiende l'Ocan Antarctique, ont t maintenues en culture au laboratoire,sous des intensits himineuss variant de 140 2 µE m^–2s^–2. The production versus intensite lumineuse, tabliespar cellule et par unit de chlorophylle a, suggrent, selonla mthode de Richardson et al. (1983), une adaptation des espcesantarctiques la lumire par variation de la taille (estimepar la valeur I_k) et du nombre (volution de la pente initiale) des units photosynthtiques. Au plan ecologique, les valeursdes paramtres photosynthetiques en lumire Bmitante et en lumiresaturante sont faibles, caractristiques des algues ‘d'ombre’:concentration en chlorophylle a et productivit par celluleplus fortes quand les espces sont acclimates a des faiblesintensites lumineuses. Ce travail met ainsi en vidence I'importancedu facteur lumineux par rapport la basse temprature sur laproduction primaire de l'Ocan Antarctique.     

Limitations on the Analysis of Acetylene Reduction by Soybean at Low Levels of Acetylene

The analysis of acetylene reduction at low concentrations ofacetylene involves a number of assumptions and both technicaland kinetic complexities. The major difficulty in convertingacetylene reduction rates to apparent N₂ reduction rates isdetermining the K_m for acetylene in the presence of N₂. Thissubstrate competition is dominant over diffusion limitationeffects, but both introduce equivalent deviations in the observedK_m. Because N₂ is a non-linear partial competitive inhibitorof acetylene reduction, correction for its presence is difficult.Two further complications are introduced by the non-linear responseof nitrogenase to acetylene concentration even in the absenceof N₂, and changes in the apparent K_m of acetylene and K₁ ofN₂ as a function of other variables in the enzyme assay. Itis proposed that transient analysis may be used for measurementof diffusion coefficients and calculations of possible diffusionlimitations. It is demonstrated that one proposed model forestimating diffusion limitation (Denison et al., 1983, PlantPhysiology 73, 648–51) confounds substrate competitionwith diffusion limitation. Acetylene reduction, nitrogen fixation, diffusion limitation     

Floodwater Oxygen Content, Ethylene Production and Lenticel Hypertrophy in Flooded Mango (Mangifera indica L.) Trees

A system was developed to test the effects of floodwater O₂concentration on ethylene evolution and stem lenticel hypertrophy,and the effects of exogenous ethylene on stem lenticel hypertrophyin mango (Mangifera indica L.) trees. Dissolved O₂ concentrationsof 1–7x10^–9 m³ m^–3 generally resulted in hypertrophyof stem lenticels within about 6 d of flooding, whereas floodwaterO₂ concentrations of 13–15 x 10^–9 m³ m^–3 delayedhypertrophy until about day 9. After 14d of flooding, therewere more than twice the number of hypertrophied lenticels pertree with floodwater O₂ concentrations of 1–7 x 10^–9m³ m^–3 than with floodwater O₂ concentrations of 15 x10^–9 m³ m^–3. Ethylene evolution from stem tissueimmediately above the floodline increased 4- to 8-fold in treesexposed to floodwater O₂ concentrations of 1–2 x 10^–9m³ m^–3, increased 2-fold for trees exposed to floodwaterO₂ concentrations of 6–7 x 10^–9 m³ m^–3, butremained constant with floodwater O₂ concentrations of 13–15x 10^–9 m³ m^–3. Plants maintained in highly oxygenatedfloodwater (13–15 x 10^–9 m³ m^–3), and givenexogenous ethylene developed many hypertrophied lenticels, whereasplants in highly oxygenated water and not given ethylene developedfewer or nohypertrophied lenticels. These data suggest thatethylene plays a role in promotion of stem lenticel hypertrophyin flooded mango trees, and that floodwater dissolved oxygenconcentration can regulate stem lenticel hypertrophy and ethyleneevolution in this species. Key words: Flooding, hypoxia, hypertrophic cell swelling