Abstract
Marine cyanobacteria are infected by phages whose genomes encode ferredoxin (Fd) electron carriers. These Fds are thought to redirect the energy harvested from light to phage-encoded oxidoreductases that enhance viral fitness, but it is unclear how the biophysical properties and partner specificities of phage Fds relate to those of photosynthetic organisms. Here, results of a bioinformatics 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 the cyanobacterium Prochlorococcus marinus, revealed high levels of similarity to cyanobacterial Fds (root mean square deviations of ≤0.5 Å). Additionally, pssm2-Fd exhibited a low midpoint reduction potential (-336 mV versus a standard hydrogen electrode), similar to other photosynthetic Fds, although it had lower thermostability (Tm = 28 °C) than did many other Fds. When expressed in an Escherichia coli strain deficient in sulfite assimilation, pssm2-Fd complemented bacterial growth when coexpressed with a P. marinus sulfite reductase, revealing that pssm2-Fd can transfer electrons to a host protein involved in nutrient assimilation. The high levels of structural similarity with cyanobacterial Fds and reactivity with a host sulfite reductase suggest that phage Fds evolved to transfer electrons to cyanobacterially encoded oxidoreductases.
Highlights
Prochlorococcus marinus is thought to be the most prevalent photosynthetic organism on Earth, with a global abundance of 1027 cells [1], making it a key player in biogeochemical processes
We determined the first structure of a cyanophage Fd, which has high structural similarity to cyanobacterial Fds. We show that this Fd has a low midpoint potential characteristic of photosynthetic Fds, we establish that this protein has a low thermostability (Tm = 28 °C), and we show that it supports electron transfer (ET) to a host oxidoreductase involved in sulfur assimilation. These results suggest that viral Fds support ET to cyanobacterially encoded oxidoreductases, and they extend our understanding about the ways in which viruses interact with cyanobacterial hosts
sequence similarity network (SSN) represent a simple way to visualize how protein paralogs relate to one another in their primary structure, and they provide a frame of reference for establishing how phageencoded proteins might have evolved from host proteins
Summary
Prochlorococcus marinus is thought to be the most prevalent photosynthetic organism on Earth, with a global abundance of 1027 cells [1], making it a key player in biogeochemical processes. It is not known how the primary structure and biophysical properties of phage Fds compare with those of host Fds. In this study, we used a sequence similarity network (SSN) analysis to examine how cyanophage and cyanobacterial Fds relate to one another.
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