Cyanobacteria play a key role in marine photosynthesis, which contributes to the global carbon cycle and the world oxygen supply. A decade ago, marine cyanophages were discovered to carry photosystem II (PSII) genes as well as genes from the electron transport chain. It was suggested that these genes increase phage fitness. More recently, cyanobacterial core photosystem I (PSI) genes were identified on environmental DNA thought to come from viruses. These genes, including psaA, B, C, D, E, Kand a unique J and F fusion, form a unique cluster on cyanophage genomes, suggestive of selection for a distinct function in virus reproduction. Furthermore, the seven proteins encoded by the viral genes are potentially sufficient for forming an intact monomeric PSI complex. However, prior tothis study, no virus with PSI genes had been isolated. Using a screening scheme devised specifically to detect these phages, we isolated a phage from phage-concentrates collected from the Line Islands (central Pacific Ocean) with ProchlorococcusMIT9515 as host. The existence of the PSI gene-cluster was confirmed by whole genome sequencing. To check whether phage PSI genes are expressed, transcript levels of phage psaA and psaB genes were determined during infection by RT-PCR and PSI peptides were detected by mass spectrometry analysis.
