Abstract
The enzyme isocitrate dehydrogenase (ICDH; EC 1.1.1.42) catalyzes the oxidative decarboxylation of isocitrate, to produce 2-oxoglutarate. The incompleteness of the tricarboxylic acids cycle in marine cyanobacteria confers a special importance to isocitrate dehydrogenase in the C/N balance, since 2-oxoglutarate can only be metabolized through the glutamine synthetase/glutamate synthase pathway. The physiological regulation of isocitrate dehydrogenase was studied in cultures of Prochlorococcus sp. strain PCC 9511, by measuring enzyme activity and concentration using the NADPH production assay and Western blotting, respectively. The enzyme activity showed little changes under nitrogen or phosphorus starvation, or upon addition of the inhibitors DCMU, DBMIB and MSX. Azaserine, an inhibitor of glutamate synthase, induced clear increases in the isocitrate dehydrogenase activity and icd gene expression after 24 h, and also in the 2-oxoglutarate concentration. Iron starvation had the most significant effect, inducing a complete loss of isocitrate dehydrogenase activity, possibly mediated by a process of oxidative inactivation, while its concentration was unaffected. Our results suggest that isocitrate dehydrogenase responds to changes in the intracellular concentration of 2-oxoglutarate and to the redox status of the cells in Prochlorococcus.
Highlights
The marine cyanobacterium Prochlorococcus [1,2] has become an important model marine microbe for ecological studies since its discovery [3,4], because of its abundance and significant contribution to global primary production [5]
Concluding remarks The present is the first study analyzing in detail the regulation of isocitrate dehydrogenase (ICDH) and its relationship with the intracellular pool of 2-OG, under a number of conditions representative of the actual challenges faced by natural Prochlorococcus populations
These results suggest that iron is a paramount nutrient for Prochlorococcus, and its complete absence can not be compensated even by the adaptive mechanisms developed by this group of cyanobacteria
Summary
The marine cyanobacterium Prochlorococcus [1,2] has become an important model marine microbe for ecological studies since its discovery [3,4], because of its abundance and significant contribution to global primary production [5]. There is a clear need for in vivo studies addressing the physiology of Prochlorococcus [18,19], in order to further understand the underpinnings of differences among ecotypes, which might illuminate the reasons explaining the tremendous ecological success of this organism. This led our team to study the metabolism of nitrogen assimilation in several Prochlorococcus strains [17,20,21,22,23,24,25]. The same study showed that marine Synechococcus and Prochlorococcus strains do lack both enzymes These groups seem to be the only cyanobacteria which possess an incomplete TCA cycle. NATL2A, annotated as di/tricarboxilate transporter), this possibility seems improbable, due to the scarcity of organic molecules in the ocean and to the high concentration of heterotrophic bacteria, which would outcompete Prochlorococcus in scavenging such compounds
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