Distribution of isoforms of ferredoxin-NADP reductase (FNR) in cyanobacteria in two growth conditions.

Ferredoxin-NADP reductase (FNR) transfers reducing equivalents between ferredoxin and NADP(H) in the photosynthetic electron transport chains of chloroplasts and cyanobacteria. In most cyanobacteria, FNR is coded by a single petH gene. The structure of FNR in photosynthetic organisms can be constituted by FAD-binding and NADPH-binding domains (FNR-2D), or by these and an additional N-terminal domain (FNR-3D). In this article, biochemical evidence is provided supporting the induction of FNR-2D by iron or combined nitrogen deficiency in the cyanobacteria Synechocystis PCC 6803 and Anabaena variabilis ATCC 29413. In cell extracts of these cyanobacteria, most of FNR was associated to phycobilisomes (PBS) or phycocyanin (PC), and the rest was found as free enzyme. Free FNR activity increased in both cyanobacteria under iron stress and during diazotrophic conditions in A. variabilis. Characterization of FNR from both cyanobacteria showed that the PBS-associated enzyme was FNR-3D and the free enzyme was mostly a FNR-2D isoform. Predominant isoforms in heterocysts of A. variabilis were FNR-2D; where its N-terminal sequence lacked an initial (formyl)methionine. This means that FNR-3D is targeted to thylakoid membrane, and anchored to PBS, and FNR-2D is found as a soluble protein in the cytoplasm, when iron or fixed nitrogen deficiencies prevail in the environment. Moreover, given that Synechocystis and Anabaena variabilis are dissimilar in genotype, phenotype and ecology, the presence of these two-domain proteins in these species suggests that the mechanism of FNR induction is common among cyanobacteria regardless of their habitat and morphotype.