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Bradley, JM, Svistunenko DA, Pullin J, Hill N, Stuart RK, Palenik B, Wilson MT, Hemmings AM, Moore GR, Le Brun NE.  2019.  Reaction of O-2 with a diiron protein generates a mixed-valent Fe2+/Fe3+ center and peroxide. Proceedings of the National Academy of Sciences of the United States of America. 116:2058-2067.   10.1073/pnas.1809913116   AbstractWebsite

The gene encoding the cyanobacterial ferritin SynFtn is up-regulated in response to copper stress. Here, we show that, while SynFtn does not interact directly with copper, it is highly unusual in several ways. First, its catalytic diiron ferroxidase center is unlike those of all other characterized prokaryotic ferritins and instead resembles an animal H-chain ferritin center. Second, as demonstrated by kinetic, spectro-scopic, and high-resolution X-ray crystallographic data, reaction of O-2 with the di-Fe2+ center results in a direct, one-electron oxidation to a mixed-valent Fe2+/Fe3+ form. Iron-O-2 chemistry of this type is currently unknown among the growing family of proteins that bind a diiron site within a four alpha-helical bundle in general and ferritins in particular. The mixed-valent form, which slowly oxidized to the more usual di-Fe3+ form, is an intermediate that is continually generated during mineralization. Peroxide, rather than superoxide, is shown to be the product of O-2 reduction, implying that ferroxidase centers function in pairs via long-range electron transfer through the protein resulting in reduction of O-2 bound at only one of the centers. We show that electron transfer is mediated by the transient formation of a radical on Tyr40, which lies similar to 4 angstrom from the diiron center. As well as demonstrating an expansion of the iron-O-2 chemistry known to occur in nature, these data are also highly relevant to the question of whether all ferritins mineralize iron via a common mechanism, providing unequivocal proof that they do not.

Stuart, RK, Bundy R, Buck K, Ghassemain M, Barbeau K, Palenik B.  2017.  Copper toxicity response influences mesotrophic Synechococcus community structure. Environmental Microbiology. 19:756-769.   10.1111/1462-2920.13630   AbstractWebsite

Picocyanobacteria from the genus Synechococcus are ubiquitous in ocean waters. Their phylogenetic and genomic diversity suggests ecological niche differentiation, but the selective forces influencing this are not well defined. Marine picocyanobacteria are sensitive to Cu toxicity, so adaptations to this stress could represent a selective force within, and between, species', also known as clades. Here, we compared Cu stress responses in cultures and natural populations of marine Synechococcus from two co-occurring major mesotrophic clades (I and IV). Using custom microarrays and proteomics to characterize expression responses to Cu in the lab and field, we found evidence for a general stress regulon in marine Synechococcus. However, the two clades also exhibited distinct responses to copper. The Clade I representative induced expression of genomic island genes in cultures and Southern California Bight populations, while the Clade IV representative downregulated Fe-limitation proteins. Copper incubation experiments suggest that Clade IV populations may harbour stress-tolerant subgroups, and thus fitness tradeoffs may govern Cu-tolerant strain distributions. This work demonstrates that Synechococcus has distinct adaptive strategies to deal with Cu toxicity at both the clade and subclade level, implying that metal toxicity and stress response adaptations represent an important selective force for influencing diversity within marine Synechococcus populations.