Closing the gaps on the viral photosystem‐ I   psa DCAB gene organization

Marine photosynthesis is largely driven by cyanobacteria, namely S ynechococcus and P rochlorococcus . Genes encoding for photosystem ( PS ) I and II reaction centre proteins are found in cyanophages and are believed to increase their fitness. Two viral PSI gene arrangements are known, psaJF→ C →A→ B → K → E → D and psaD→ C →A→ B . The shared genes between these gene cassettes and their encoded proteins are distinguished by %G + C and protein sequence respectively. The data on the psaD→ C →A→ B gene organization were reported from only two partial gene cassettes coming from Global Ocean Sampling stations in the P acific and I ndian oceans. Now we have extended our search to 370 marine stations from six metagenomic projects. Genes corresponding to both PSI gene arrangements were detected in the P acific, I ndian and A tlantic oceans, confined to a strip along the equator (30°N and 30°S). In addition, we found that the predicted structure of the viral PsaA protein from the psaD→ C → A → B organization contains a lumenal loop conserved in PsaA proteins from S ynechococcus , but is completely absent in viral PsaA proteins from the psaJF→ C → A → B → K → E → D gene organization and most P rochlorococcus strains. This may indicate a co‐evolutionary scenario where cyanophages containing either of these gene organizations infect cyanobacterial ecotypes biogeographically restricted to the 30°N and 30°S equatorial strip.