Activity and functional properties of the isocitrate lyase in the cyanobacterium Cyanothece sp. PCC 7424

Abstract

Cyanobacteria are ubiquitous photoautotrophs that assimilate atmospheric CO2 as their main source of carbon. Several cyanobacteria are known to be facultative heterotrophs that are able to grow on diverse carbon sources. For selected strains, assimilation of organic acids and mixotrophic growth on acetate has been reported for decades. However, evidence for the existence of a functional glyoxylate shunt in cyanobacteria has long been contradictory and unclear. Genes coding for isocitrate lyase (ICL) and malate synthase were recently identified in two strains of the genus Cyanothece, and the existence of the complete glyoxylate shunt was verified in a strain of Chlorogloeopsis fritschii. Here, we report that the gene PCC7424_4054 of the strain Cyanothece sp. PCC 7424 encodes an enzymatically active protein that catalyses the reaction of ICL, an enzyme that is specific for the glyoxylate shunt. We demonstrate that ICL activity is induced under alternating day/night cycles and acetate-supplemented cultures exhibit enhanced growth. In contrast, growth under constant light did not result in any detectable ICL activity or enhanced growth of acetate-supplemented cultures. Furthermore, our results indicate that, despite the presence of a glyoxylate shunt, acetate does not support continued heterotrophic growth and cell proliferation. The functional validation of the ICL is supplemented with a bioinformatics analysis of enzymes that co-occur with the glyoxylate shunt. We hypothesize that the glyoxylate shunt in Cyanothece sp. PCC 7424, and possibly other nitrogen-fixing cyanobacteria, is an adaptation to a specific ecological niche and supports assimilation of nitrogen or organic compounds during the night phase.

Proteome Expression and Survival Strategies of a Proteorhodopsin-ContainingVibrioStrain under Carbon and Nitrogen Limitation

“…One notable expression signal in C-limited stationary phase was increased utilization of the glyoxylate shunt via isocitrate lyase AceA, which bypasses CO 2 -losing steps in the tricarboxylic acid (TCA) cycle, thereby conserving fixed carbon. Isocitrate lyase also showed a tendency to higher expression in the dark than the light (though it did not pass our test for significantly differential light/dark expression), consistent with expression patterns found in cyanobacteria ( 42 ) but opposite to those in proteorhodopsin-containing Dokdonia sp. MED134 ( 15 ).…”

Phosphoglycolate salvage in a chemolithoautotroph using the Calvin cycle

“…Synechocystis , the only bacterial photoautotroph for which phosphoglycolate salvage has been physiologically characterized ( 5 , 8 ), probably lacks malate synthase and hence, cannot operate the malate cycle ( 38 , 39 ). However, the presence of malate synthase has been confirmed in some cyanobacteria, leading to the suggestion that they may use the malate cycle ( 40 – 43 ). We leave it for future studies to explore the occurrence of the malate cycle and other phosphoglycolate salvage routes in cyanobacteria and chemolithoautotrophs.…”

Photorespiration pathways in a chemolithoautotroph

“…Synechocystis, the only bacterial photoautotroph for which photorespiration has been physiologically characterized (5,8), probably lacks malate synthase and hence cannot operate the malate cycle (32,33). However, the presence of malate synthase has been confirmed in some cyanobacteria (34)(35)(36)(37), leading to the suggestion that they may use the malate cycle. We leave it for future studies to explore the occurrence of the malate cycle and other photorespiration routes in cyanobacteria and chemolithoautotrophs.…”