Sll1263, a Unique Cation Diffusion Facilitator Protein that Promotes Iron Uptake in the Cyanobacterium Synechocystis sp. Strain PCC 6803

Cyanobacteria are known to survive in iron-deficient environments, but the ways in which they acquire Fe and acclimate are not completely understood. Here we report a novel gene sll1263 that is required for Synechocystis sp. strain PCC 6803 to grow under iron-deficient conditions. sll1263 encodes a putative cation diffusion facilitator protein (CDF) that shows 50% amino acid similarity with ferrous iron efflux protein (FieF) of heterotrophic bacteria. In bacteria, the gene product is involved in metal export from the cell, but in Synechocystis sll1263 plays a role in iron uptake. The results show that expression of sll1263 was induced by iron-deficient conditions and its inactivation significantly decreased the growth rate of an sll1263(-) mutant. Other genes known to be required for Fe acquisition were also strongly up-regulated in the mutant even in the presence of high Fe. Overexpression of sll1263 increased growth under iron deficiency but reduced growth under high-iron stress, suggesting that the gene product was involved in iron uptake rather than detoxification. Expression of FieF in the sll1263(-) mutant was unable to rescue the Fe-deficient phenotype, but Sll1263 completely restored it. Measurements of cellular iron content and the iron uptake rate showed that they were significantly less in the sll1263(-) mutant than in the wild type, consistent with a role for sll1263 in iron uptake. We hypothesize that the low-iron habitats and high-iron requirements of cyanobacteria may be the reason why cyanobacterial CDF protein functions in Fe uptake and not efflux as in non-photosynthetic bacteria.