Flavodoxin accumulation contributes to enhanced cyclic electron flow around photosystem I in salt‐stressed cells of Synechocystis sp. strain PCC 6803

The salt‐regulated accumulation of flavodoxin encoded by the isiB gene and its possible function were investigated in the cyanobacterium Synechocystis sp. strain PCC 6803. In Northern blot experiments, a slight increase of the isiB‐specific mRNA was observed in salt‐shocked and salt‐acclimated cells. High levels of flavodoxin protein were detected in cells acclimated to 342 mM NaCl. In order to analyze the function of flavodoxin in cyanobacterial salt acclimation, an insertion null mutant of isiB was constructed. It was possible to adapt this mutant to raised salt concentrations and, as expected, to low iron contents. Salt‐acclimated cells of wild type (WT) Synechocystis display increased activity of photosystem I (PSI), primarily used for increased cyclic electron transport capacity (Jeanjean et al. 1993, Plant Cell Physiol 34: 1073–1079). In salt‐acclimated cells of the flavodoxin null mutant, the level of cyclic electron flow was lower than in wild type cells. It was concluded that flavodoxin plays a role as an alternative electron carrier, used for cyclic electron flow in salt‐treated Synechocystis cells.