Abstract
Two distinct types of ferritin-like molecules often coexist in bacteria, the heme binding bacterioferritins (Bfr) and the non-heme binding bacterial ferritins (Ftn). The early isolation of a ferritin-like molecule from Pseudomonas aeruginosa suggested the possibility of a bacterioferritin assembled from two different subunits [Moore, G. R., et al. (1994) Biochem. J. 304, 493-497]. Subsequent studies demonstrated the presence of two genes encoding ferritin-like molecules in P. aeruginosa, designated bfrA and bfrB, and suggested that two distinct bacterioferritins may coexist [Ma, J.-F., et al. (1999) J. Bacteriol. 181, 3730-3742]. In this report, we present structural evidence demonstrating that the product of the bfrA gene is a ferritin-like molecule not capable of binding heme that harbors a catalytically active ferroxidase center with structural properties similar to those characteristic of bacterial and archaeal Ftns and clearly distinct from those of the ferroxidase center typical of Bfrs. Consequently, the product of the bfrA gene in P. aeruginosa is a bacterial ferritin, which we propose should be termed Pa FtnA. These results, together with the previous characterization of the product of the bfrB gene as a genuine bacterioferritin (Pa BfrB) [Weeratunga, S. J., et al. (2010) Biochemistry 49, 1160-1175], indicate the coexistence of a bacterial ferritin (Pa FtnA) and a bacterioferritin (Pa BfrB) in P. aeruginosa. In agreement with this idea, we also obtained evidence demonstrating that release of iron from Pa BfrB and Pa FtnA is likely subject to different regulation in P. aerugionsa. Whereas the efficient release of iron stored in Pa FtnA requires only the input of electrons from a ferredoxin NADP reductase (Pa Fpr), the release of iron stored in Pa BfrB requires not only electron delivery by Pa Fpr but also the presence of a "regulator", the apo form of a bacterioferritin-associated ferredoxin (apo Pa Bfd). Finally, structural analysis of iron uptake in crystallo suggests a possible pathway for the internalization of ferroxidase iron into the interior cavity of Pa FtnA.