CLOSING THE GAPS ON THE VIRAL PHOTOSYSTEM-I psaDCAB GENE ORGANIZATION

Marine photosynthesis is largely driven by cyanobacteria, mainly Synechococcus and Prochlorococcus. Cyanophages can alter cyanobacterial populations and therefore affect the global carbon cycle and oxygen supply. Genes encoding for photosystem II (PSII) and I (PSI) reaction center 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 is scarce and was obtained from only two Global Ocean Sampling (GOS) stations in the Pacific and Indian Oceans. In order to expand our knowledge regarding the differences between the viral PSI cassettes and their possible hosts we conducted a comprehensive bioinformatic search in six metagenomic datasets. We surveyed 370 marine stations, resulting in new positive stations for viral-PSI genes in the Pacific and Indian Oceans and, for the first time, also in the Atlantic Ocean. The stations containing the psaD→C→A→B gene arrangement (as well as the longer gene arrangement) are confined to a strip along the equator (± 30^o). In addition, we succeeded in filling the partially assembled psaD→C→A→B cassette sequence. According to our metagenomic findings viral photosynthetic genes are found solely in the two previously described cassettes. Protein structure modeling of both viral PsaA show structural differences, the psaD→C→A→B PsaA protein being more similar to Synechococcus. We propose that phages carrying PSI genes in the psaD→C→A→B cassette might infect Synechococcus or low light (LL), clade IV Prochlorococcus which inhabit the oceans between 30⁰N, 30⁰S.