A redox switch allows binding of ferrous and ferric ions in the cyanobacterial iron binding protein FutA fromProchlorococcus

The marine cyanobacteriumProchlorococcusis a main contributor to global photosynthesis, whilst being limited by iron availability. Cyanobacterial genomes typically encode two different types of FutA iron binding proteins: periplasmic FutA2 ABC transporter subunits bind ferric (Fe3+), while cytosolic FutA1 binds ferrous (Fe2+). Owing to their small size and their economized genomeProchlorococcusecotypes typically possess a singlefutAgene. How the encoded FutA protein might bind different Fe oxidation states was previously unknown. Here we use structural biology techniques at room temperature to probe the dynamic behavior of FutA. Neutron diffraction confirmed four negatively charged tyrosinates, that together with a solvent molecule coordinate iron in trigonal bipyramidal geometry. Positioning of the positively charged Arg103 side chain in the second coordination shell was consistent with an overall charge-neutral ferric binding state in structures determined by neutron diffraction and serial femtosecond crystallography.Conventional rotation X-ray crystallography using a home source revealed X-ray induced photoreduction of the iron center with observation of the ferrous binding state; here, an additional positioning of the Arg203 side chain in the second coordination shell maintained an overall charge neutral ferrous binding site. Room temperature dose series using serial synchrotron crystallography and an XFEL X-ray pump-probe approach capture the transition between ferric and ferrous states, revealing how Arg203 operates as a switch to accommodate the different iron oxidation states. This switching ability of theProchlorococcusFutA protein may reflect ecological adaptation by genome streamlining and loss of specialized FutA proteins.Significance StatementOceanic primary production by marine cyanobacteria is a main contributor to carbon and nitrogen fixation.Prochlorococcusis the most abundant photosynthetic organism on Earth, with an annual carbon fixation comparable to the net global primary production from agriculture. Its remarkable ecological success is based on the ability to thrive in low nutrient waters. To manage iron limitation,Prochlorococcuspossesses the FutA protein for iron uptake and homeostasis. We reveal a switch in the FutA protein that allows it to accommodate binding of iron in either the ferric (Fe3+) or ferrous (Fe2+) state using structural biology techniques at room temperature and provide a plausible mechanism for FutA as a bifunctional redox state sensing protein.