Photorespiratory metabolism in the cyanobacterial model Synechocystis sp. strain PCC 6803: A systems biology approach

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The inorganic carbon-concentrating mechanism enables cyanobacterial growth in low CO2/high O2 environments and was until recently thought sufficient to suppress the oxygenase reaction of cyanobacterial ribulose-1,5-bisphosphate-carboxylase/-oxygenase (Rubisco). However, photorespiratory metabolism and glycolate detoxification were shown to be essential for cyanobacteria and were both endosymbiontically conveyed into the plant kingdom. We recently established Synechocystis sp. PCC 6803 as a cyanobacterial model for systems analyses at the transcriptome, metabolome and fluxome levels under conditions enhancing photorespiration by low inorganic carbon (Ci) availability (LC), in contrast to high Ci availability (HC), a condition that largely suppresses photorespiratory responses. These tools, specifically the flux phenotyping, allowed the formal demonstration of photorespiration and a photorespiratory burst upon shift from HC to LC using a glycolate accumulating ΔglcD1 glycolate dehydrogenase mutant. The same experiments indicated that transfer to LC conditions activates additional/alternative CO2 assimilation likely via phosphoenolpyruvate (PEP)-carboxylase, a reaction that is also utilized by higher plants. Here we propose to study the interaction of photorespiration with central metabolism by 13C-flux probing of glycolate dehydrogenase deletion mutants in combination with gene deletions that interfere with the CCM and the carbon-fixing PEP carboxylase pathway. The study will reveal, if and how the two main carbon fixing pathways interact.