Metabolism of 2-carboxyarabinitol l-phosphate (CA l-P), an endogenous inhibitor of ribulose. bisphosphate carboxylase/ oxygenase, occurs in the light. A soluble protein fraction which metabolized CA 1-P in the presence of NADPH was isolated from tobacco chloroplasts. A similar fraction from spinach exhibited much lower activity. The activity in tobacco extracts was stable overnight at 4°C but its maintenance during storage required dithiothreitol. The tobacco protein responsible for CA 1-P metabolism was partially purified by ion-exchange FPLC of stromal extracts. The requirements for NADPH and dithiothreitol for activity of this protein suggest a mechanism for the light-dependent control of CA 1-P levels in plants.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to International Journal of Plant Sciences.A two-tiered, nested molecular phylogenetic study of panicoid grasses to explore character state transitions between the C 3 and C 4 adaptive syndromes is presented. A broad survey of 92 panicoid species was sampled for the grass-specific insert sequence in the chloroplast RNA polymerase locus (rpoC2), combining published and unpublished sequences. This portion of the study also included an intensive phylogenetic investigation of one clade of seven species that included Steinchisma hians, which is notable for exhibiting intermediacy between the C 3 and C 4 photosynthetic types. Both rpoC2 data and previously published sequences of the F subunit of an NADH-dependent dehydrogenase were analyzed together for this small group. A rigorous phylogenetic investigation of S. hians and 13 other species of Panicoideae included in the broad survey was then performed with sequences of both rpoC2 and the externally transcribed spacer region of the nuclear ribosomal repeat. These 14 species were selected to maximize representation among photosynthetic subtypes. Combined analysis resolved single origins of two photosynthetic subtypes. A reversion of C 4 to C 3 photosynthesis during the evolution of the lineage that includes S. hians is identified. These and other recent results indicate that repeated reversions from C 4 to C 3 have occurred. The C 3 species Panicum laxum has a strongly supported sister group relationship to S. hians (C 3 -C 4 ). The most parsimonious interpretation is that S. hians represents an incipient reversal from C 3 to C 4 photosynthesis, beginning with the capacity to compartmentalize photorespiratory metabolism in the bundle sheath tissue.

The catalytic degradation of 2-carboxyarabinitol 1-phosphate (CA 1-P), a naturally occurring inhibitor of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), was investigated by chromatographic and spectroscopic analyses of the reaction products. Carboxy-labeled [14C]CA 1-P was incubated with a partially purified tobacco (Nicotiana rustica) chloroplast protein that has been shown previously to catalyze metabolism of CA 1-

The regulation of 2-carboxyarabinitol 1-phosphatase (CA 1-Pase) by phosphorylated effectors was studied with enzyme purified from tobacco (Nicotiana tabacum) leaves. CA 1-Pase activity was most stimulated by fructose 1,6-bisphosphate, exhibiting an Ao.5 value of 1.9 millimolar and a 10-fold enhancement of catalysis. With nbulose-1,5-bisphosphate, the AO.5 was 0.6 millimolar, and maximal stimulation of activity was 5.3-fold. Among the monophosphates, 3-phosphoglycerate and phosphoglycolate were more potent positive effectors than glyceraldehyde 3-phosphate, glucose 1-phosphate, glucose 6-phosphate, and dihydroxyacetone phosphate. Stimulation of CA 1-Pase by ribulose-1,5-bisphosphate and fructose 1,6-bisphosphate increased Vmax but did not appreciably alter Km (2-carboxyarabinitol 1-phosphate) values. Inorganic phosphate appeared to inhibit CA 1-Pase noncompetitively with respect to 2-carboxyarabinitol 1-phosphate, exhibiting a K, of 0.3 millimolar. The results suggest that these positive and negative effectors bind to a regulatory site on CA 1-Pase and may have a physiological role in the light regulation of this enzyme. Related experiments with CA 1-Pase inactivated by dialysis in the absence of dithiothreitol show that partial reactivation can be achieved in the presence of a range of reducing reagents, including dithiothreitol, cysteine, and reduced glutathione. This could imply an ancillary involvement of sulfhydryl reduction during light activation of CA 1-Pase in vivo. The enzyme was thermally stable up to 350C, in contrast to ribulose-1,5-bisphosphate carboxylase/oxygenase activase which lost activity above 300C. The activation energy for CA 1-Pase was calculated to be 56.14 kilojoules per mole.

During a shipboard expedition to Andros Island (Bahamas), photosynthetic measurements for the macroalgae Cladophoropsis membranacea (Chlorophyta), Dilophus guineensis, Turbinaria turbinata, and Lobophora variegata (Phaeophyta), and Laurencia papillosa (Rhodophyta), taken directly from their marine habitat, showed that only Cladophoropsis was saturated at seawater inorganic carbon levels (2.5 mM). The photosynthetic k0.5 values for inorganic carbon ranged from 1.1 to 3.2 mM. Decreasing the pH at 2.5 mM inorganic carbon, and thus enhancing the CO2 by 30-fold, only slightly increased photosynthesis, suggesting that bicarbonate was the major assimilated form of inorganic carbon. At 2.5 mM inorganic carbon, only Lobophora exhibited a Warburg effect on photosynthesis (49%), but at 0.5 mM, Turbinaria and Laurencia were also inhibited by O2. Ribulosebisphosphate carboxylase–oxygenase appeared to be the predominant carboxylation enzyme, but in Dilophus and Laurencia extracts, its activity was rivaled by phosphoenolpyruvate carboxylase and carboxykinase. Malate pools were detected in Turbinaria and Laurencia, and in the latter they were greater at night than during the day. However, this diel fluctuation was too small to implicate crassulacean acid metabolism. The data indicate that the bicarbonate concentration in seawater is insufficient to overcome O2 inhibition effects on photosynthesis, unless the macroalga has some form of CO2 concentrating system, based on bicarbonate uptake or C4 acid metabolism. In addition, the inorganic carbon in seawater may be a nutrient limiting the photosynthesis and productivity of certain macroalgae.

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Carboxyarabinitol 1-phosphatase (2-carboxy-D-arabinitol 1-phosphate phosphohydrolase), a chloroplast enzyme that metabolizes the naturally occurring inhibitor of ribulose-1,5-bisphosphate carboxylase/oxygenase, was isolated from tobacco (Nicotiana tabacum) leaves. The enzyme was purified more than 3500-fold using a protocol that included ammonium sulfate fractionafton and gel filtration, ion-exchange, and hydrophobic interaction chromatography. Analysis of the final preparation by sodium dodecyl sulfate polyacrylamide gel electrophoresis revealed the presence of a single polypeptide with a molecular mass of 53 kilodaltons. The enzyme exhibited an apparent Km (carboxyarabinitol 1-phosphate) of 33 micromolar and a pH optimum of 7.5. Enzyme activity did not require divalent cations and was unaffected by the metal chelators EDTA and cysteine. Carboxyarabinitol 1-phosphatase activity was inhibited by zinc, copper and molybdate and stimulated by sulfate. Chloroplast metabolites that affected activity included inorganic phosphate and ATP, which were inhibitory, and ribulose-1,5-bisphosphate, fructose-1,6-bisphosphate and NADPH which stimulated activity 2.5-fold. Activation of carboxyarabinitol 1-phosphatase activity by these positive effectors, together with the previously reported requirement for dithiothreitol, explain the light/dark modulation of carboxyarabinitol 1-phosphatase activity in vivo.phatase (I 1, 16, 17, 20) presumably by modifying the accessibility of critical sulfhydryl groups (2, 21).In the photorespiratory carbon oxidation cycle, a chloroplast phosphatase has been described that catalyzes the phosphohydrolysis ofphosphoglycolate (8, 23), one ofthe products of the ribulose-1,5-bisphosphate oxygenase reaction (4). The enzymic properties of phosphoglycolate phosphatase include inhibition of activity by ribose-5-phosphate and activation by both anions and divalent cations (13); however, aside from these factors, phosphoglycolate phosphatase does not appear to be under extensive regulatory control. This apparent lack of regulatory control may be indicative of a housekeeping function for this enzyme of rapidly catalyzing the metabolism of phosphoglycolate. The conversion of phosphoglycolate to glycolate is not of trivial importance for it prevents the accumulation of phosphoglycolate, a potent inhibitor of triose-phosphate isomerase (31), and initiates the recycling of the carbon lost in photorespiration back to the Calvin Cycle (4,8,28).In a companion study (12), a newly discovered plant phosphatase was shown to be involved in the metabolism of 2-carboxyarabinitol 1-phosphate (CA I-P2), an endogenous inhibitor of Rubisco. In the present study, we report a detailed protocol for the purification of CA l-Pase (2-carboxy-D-arabinitol 1-phosphate phosphohydrolase) from tobacco leaves and describe some of the properties of the isolated enzyme.Three phosphohydrolytic reactions occur as steps in the pathways for photosynthetic and photorespiratory carbon metabolism. In the reductive pentose phosphate cyc...

The use of microalgae as feedstocks for biodiesel is potentially limited by climatic conditions with low light and temperature levels. Monoraphidium sp. Dek19 was identified by 18S rRNA gene sequencing. This is a species indigenous to the upper Midwestern USA which grows to high densities in wastewater with bioremediation resulting in a reduction of nitrate and phosphate levels. Mesocosm pool cultures (171L) were used to characterize growth of this species in larger volumes than typical of previous lab-based studies,. The alga was shown to be cold-tolerant and to grow to workable density within 15 days at relatively low light intensities in sterilized treated effluent. It was harvested by FeCl3 flocculation and filtration, and lipid content was measured at 26% of dry weight. Pool cultures yielded enough biomass to extract lipids and transesterify them to biodiesel for testing in a stationary engine. GC analysis showed FAMEs produced from Monoraphidium sp. to have a similar fatty acid profile to soybean oil. Engine testing of this algal biodiesel in blends with petrodiesel showed