The cyanobacterium Prochlorococcus is one of the most abundant photosynthetic microorganisms of the oligotrophic ocean, where nutrients including phosphorus (P) are limited. To cope with P limitation, Prochlorococcus exploits the phosphate-specific transport (Pst) system for phosphate uptake in combination with the high-affinity capture of inorganic phosphate. Cyanophages (viruses that infect cyanobacteria) play an important role in the ocean ecosystem. Interestingly, many cyanophages carry a set of phosphate acquisition genes of host origin. In addition, previous studies have reported that an upregulated expression of phage pstS, which encodes a high-affinity phosphate binding protein, in response to P limitation. Here, we investigated the impact of phage infection on phosphate up...[ Read more ]

The cyanobacterium Prochlorococcus is one of the most abundant photosynthetic microorganisms of the oligotrophic ocean, where nutrients including phosphorus (P) are limited. To cope with P limitation, Prochlorococcus exploits the phosphate-specific transport (Pst) system for phosphate uptake in combination with the high-affinity capture of inorganic phosphate. Cyanophages (viruses that infect cyanobacteria) play an important role in the ocean ecosystem. Interestingly, many cyanophages carry a set of phosphate acquisition genes of host origin. In addition, previous studies have reported that an upregulated expression of phage pstS, which encodes a high-affinity phosphate binding protein, in response to P limitation. Here, we investigated the impact of phage infection on phosphate uptake of cyanobacteria from three aspects: First, the function of phage-encoded PstS was characterized by heterologously expressing the pstS gene from cyanophage P-SSM2 in Escherichia coli (E. coli), and its binding affinity for phosphate was compared to that of the host Prochlorococcus NATL2A. We found phage-encoded PstS to be capable of binding phosphate molecules, and its phosphate binding affinity (K_d = 1.84 μM) was lower than that of host-encoded PstS (K_d = 0.68 μM). Second, to explore the effect of phage infection on the host’s uptake capabilities of phosphate, in vivo phosphate uptake kinetics of P-SSM2 infected NATL2A cells were measured in either phosphate-replete or phosphate-limited conditions. Both the maximal uptake velocity (V_max) and the apparent Michaelis-Menten constant (K_M) for phosphate increased in P-limited conditions in response to P-SSM2 infection, suggesting a higher capacity but lower binding affinity for phosphate in the infected cells. Finally, co-infection experiments were performed by simultaneously infecting NATL2A cells with a phage with pstS (like P-SSM2) and a phage without pstS (like P-HM2) and extracellular phage numbers were estimated to show phage production. Co-infection experiments showed that the ratio between numbers of phage with pstS and phage without pstS was higher in low-phosphate conditions. In summary, this study is the first to demonstrate that phage-encoded PstS is functional. Despite lower binding affinity for phosphate, phage-encoded PstS may facilitate phosphate uptake of host cells during infection, which might confer a fitness advantage to phages in phosphate-limited environments.