Glutamine Synthetase Isoenzymes in the Green Soil Alga Stichococcus bacillaris Naeg.

1988

Plant Physiol.

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ABSTRACIT Chlorella autotrophica, a euryhaline marine alga, and Stichococcus bacillais, a salt-tolerant soil alga, grow in the presence of methionine sulfoximine (MSX), an inhibitor of glutamine synthetase, by maintaining high levels of NADPH-glutamate dehydrogenase. Nitrate reductase showed no change in MSX-adapted cells. For both species, MSX-adapted cells retained their capacity to accumulate proline in response to salinity, and in S. bacillaris no major shift was observed in the presence of MSX toward the accumulation of sorbitol. Following transfer from 33 to 150% artificial seawater (ASW), both algae exhibited increases in organic solute levels without a lag. Within 6 h of this sudden increase in salinity, the levels of proline in C. autotrophica and of proline and sorbitol in S. badillaris were similar to those found in steady state 150% ASW cultures. Following transfer from 33 to 150% ASW, S. bacillaris continued ['4Cq bicarbonate photoassimilation at a normal rate and maintained active enzymes of nitrogen assimilation. The incorporation of '4Cqphenylalanine into proteins was inhibited for about 30 minutes in MSX-free*cells and 90 minutes in MSX-adapted cells following transfer from 33 to 150% ASW; the recovery after these lag periods was almost complete.

Section: Growth Characteristics the Results Shown Inmentioning

confidence: 99%

“…Sorbitol was determined enzymically by the spectrophotometric procedure described by WilliamsAshman (30 (6). The cell suspension was disrupted as described for NADH-NR extracts.…”

mentioning

confidence: 99%

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The Relationship between Inorganic Nitrogen Metabolism and Proline Accumulation in Osmoregulatory Responses of Two Euryhaline Microalgae

Ahmad¹,

1988

Plant Physiol.

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“…The FPLC media employed were prepared according to Ahmad & Hellebust (1987, 1989 with the addition of thiol reagents (1 mM dithiothreitol, 10 mM mercaptoethanol) for all enzyme purifications except for that of the GSl isoenzyme. The salt gradient and fraction collection were as described by Ahmad & Hellebust (1987).…”

Section: Fast Protein Liquid Chromatographymentioning

confidence: 99%

Partial characterization of enzymes of nitrogen metabolism in Chlorella autotrophica Shihira & Krauss

Ahmad¹,

1993

New Phytologist

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SUMMARYChlorella autotrophica shows a major decline in the intracellular concentrations of glutamine synthetase (GS) isoenzymes, GSl and GS2, and a marked increase in the concentration of NADPH-dependent glutamate dehydrogenase (GDH) when nitrate is replaced by ammonium as the sole nitrogen source. NADH-dependent GDH shows very little change under these conditions. The apparent K^^ for ammonium of both GS isoenzymes is around 50/

“…The composition of the elution buffers and fraction collections were described previously (4). Protein content of the extracts was determined as described elsewhere (1).…”

Section: Fast Protein Liquid Chromatographymentioning

confidence: 99%

Regulation of Chloroplast Development by Nitrogen Source and Growth Conditions in a Chlorella protothecoides Strain

Ahmad

1990

Plant Physiol.

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Chlorella strain (UTEX 27) maintains optimal photosynthetic capacity when growing photoautotrophically in the presence of ammonium. Nitrate-grown photoautotrophic cells, however, show a drastic loss of chlorophyll content and nbulose-1,6-bisphosphate carboxylase/oxygenase activity, resulting in a greater than 10-fold decrease in photosynthetic capacity and growth rate. Nitrate-grown cells are not deficient in protein content, and under mixotrophic and heterotrophic conditions, the alga can utilize nitrate as well as it does ammonium. The alga metabolizes both glucose and acetate in the dark with a doubling time of 5 to 6 hours. However, its growth on acetate is inhibited by light. Ribulose-1,6-biphosphate carboxylase/oxygenase activity correlates well with photosynthetic capacity, and glucose 6-phosphate dehydrogenase and hexokinase activities are altered in a manner consistent with the availability of glucose in growing cells. The alga appears to assimilate ammonium under photoautotrophic conditions primarily via the glutamine synthetase pathway, and shows an induction of both NADH and NADPH dependent glutamate dehydrogenase pathways under mixotrophic and heterotrophic conditions. Multiple isoforms are present only for hexokinase and glucose 6-phosphate dehydrogenase. Etiolated nitrate-grown cells resume greening and increase their photosynthetic capacity after about 6 hours of incubation in the presence of ammonium under photoautotrophic conditions. Similarly, the loss of photosynthetic capacity in ammonium-grown photoautotrophic cells commence about 9 hours after their transfer to heterotrophic nitrate containing media.group. Interestingly, some of these algal strains show similar etiolation in the dark and under nitrogen limiting conditions (12,22), suggesting the presence ofcommon regulatory mechanisms. Photosynthetic carbon fixation and nitrogen assimilation are generally considered to be highly intertwined mechanisms in green plants. It has been proposed that the regulation of chloroplast development is associated with carbon and nitrogen balance (10, 23). Horrum and Schwartzbach (12) studying chloroplast development in E. gracilis, have, however, cautioned against such oversimplifications. The interactions between photosynthetic mode of carbon assimilation and nitrogen assimilation are complex, and though the two processes are largely interdependent, in part they can also be competitive in nature.It should be noted that both nitrate assimilation and photosynthetic carbon fixation are reductive processes that can potentially compete for photochemically generated electrons under phototrophic conditions. Syrett (24) has estimated that the ratio of electrons required for carbon dioxide fixation and nitrate assimilation is about 2:1. Consistent with this high demand of reducing power for nitrate assimilation is the observed nitrate-induced depression of photosynthesis in algae (17,24,26). However, we know little about the relationship between the chemical nature of the inorganic nitrogen source and th...