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Prochlorococcus phage ferredoxin: structural characterization and electron transfer to cyanobacterial sulfite reductases

By Ian J. Campbell, Jose Luis Olmos, Weijun Xu, Dimithree Kahanda, Joshua T Atkinson, Othneil N. Sparks, Mitchell D. Miller, George N. Phillips, George N Bennett, Jonathan Silberg

Posted 07 Feb 2020
bioRxiv DOI: 10.1101/2020.02.07.937771 (published DOI: 10.1074/jbc.RA120.013501)

Marine cyanobacteria are infected by phage whose genomes encode ferredoxin (Fd) electron carriers. While these Fds are thought to redirect the energy harvested from light to phage-encoded oxidoreductases that enhance viral fitness, it is not clear how the biophysical properties and partner specificities of phage Fds relate to those in photosynthetic organisms. Bioinformatic analysis using a sequence similarity network revealed that phage Fds are most closely related to cyanobacterial Fds that transfer electrons from photosystems to oxidoreductases involved in nutrient assimilation. Structural analysis of myovirus P-SSM2 Fd (pssm2-Fd), which infects Prochlorococcus marinus, revealed high similarity to cyanobacterial Fds (≤0.5 Å RMSD). Additionally, pssm2-Fd exhibits a low midpoint reduction potential (-336 mV vs. SHE) similar to other photosynthetic Fds, albeit lower thermostability (Tm = 28°C) than many Fds. When expressed in an Escherichia coli strain with a sulfite assimilation defect, pssm2-Fd complemented growth when coexpressed with a Prochlorococcus marinus sulfite reductase, revealing that pssm2-Fd can transfer electrons to a host protein involved in nutrient assimilation. The high structural similarity with cyanobacterial Fds and reactivity with a host sulfite reductase suggest that phage Fds evolved to transfer electrons to cyanobacterial-encoded oxidoreductases. ### Competing Interest Statement The authors have declared no competing interest.

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