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Significance of phosphoenolpyruvate carboxylase during ammonium assimilation: carbon isotope discrimination in photosynthesis and respiration by the N-limited green alga Selenastrum minutum

The effect of N-assimilation on the partitioning of carbon fixation between phosphoenolpyruvate carboxylase (PEPcase) and ribulose bisphosphate carboxylase/oxygenase (Rubisco) was determined by measuring stable carbon isotope discrimination during photosynthesis by an N-limited green alga, Selenastr...

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Bibliographic Details
Published in:Plant physiology (Bethesda) 1989-04, Vol.89 (4), p.1150-1157
Main Authors: Guy, R.D. (Queen's University, Kingston, Ontario, Canada), Vanlerberghe, G.C, Turpin, D.H
Format: Article
Language:English
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Summary:The effect of N-assimilation on the partitioning of carbon fixation between phosphoenolpyruvate carboxylase (PEPcase) and ribulose bisphosphate carboxylase/oxygenase (Rubisco) was determined by measuring stable carbon isotope discrimination during photosynthesis by an N-limited green alga, Selenastrum minutum (Naeg.) Collins. This was facilitated by a two process model accounting for simultaneous CO2 fixation and respiratory CO2 release. Discrimination by control cells was consistent with the majority of carbon being fixed by Rubisco. During nitrogen assimilation however, discrimination was greatly reduced indicating an enhanced flux through PEPcase which accounted for upward of 70% of total carbon fixation. This shift toward anaplerotic metabolism supports a large increase in tricarboxylic acid cycle activity primarily between oxaloacetate and alpha-ketoglutarate thereby facilitating the provision of carbon skeletons for amino acid synthesis. This provides an example of a unique set of conditions under which anaplerotic carbon fixation by PEPcase exceeds photosynthetic carbon fixation by Rubisco in a C3 organism
ISSN:0032-0889
1532-2548
DOI:10.1104/pp.89.4.1150