Novel heterocyst-specific flavodiiron proteins in Anabaena sp. PCC 7120.

Flavodiiron proteins present in many prokaryotic and some eukaryotic organisms have a capacity to protect cells against nitrosative or oxidative stress. In Anabaena sp. PCC 7120, Flv1 and Flv3 proteins are encoded by families of two genes. We demonstrate here that flv1A and flv3A genes are up-regulated in vegetative cells in low CO₂ and high light conditions. In contrast, flv1B and flv3B genes are expressed in N₂-fixing conditions and corresponding proteins are located exclusively in heterocysts. It is suggested that Flv1B and Flv3B protect enzymes of N₂-fixation in heterocysts of Anabaena 7120 by reducing molecular oxygen directly to water.