Cyanobacteria play a key role in marine photosynthesis, contributing almost 50% of primary production in oligotrophic regions of the ocean. Marine cyanophages were recently discovered to carry photosystem II (PSII) genes, and it was suggested that these genes increase phage fitness by helping the phages to maintain photosynthesis in the infected bacterial cells. We recently showed evidence for the presence of photosystem I (PSI) genes in genomes of marine cyanophages [Sharon et al. 2009 Nature 461, 258-262]. Cyanobacterial core PSI gene cassettes, containing psaJFABCDEK, or psaDCAB gene cassettes forms unique clusters in cyanophage genomes, suggestive of selection for a distinct function in virus reproduction. Potentially, the proteins encoded by the viral genes are sufficient for forming intact monomeric PSI complexes. Projection of viral predicted peptides on the cyanobacterial PSI crystal structure suggests that the viral PSI components provide a unique way for funneling reducing power from respiratory and other electron transfer chains to PSI, therefore bypassing the need to rely solely on reducing power from the photosystem electron transfer chain.The main goals of this proposal are:(1) To determine how much of oceanic photosynthesis is actually performed with viral proteins.(2) To establish a model system to understand the role of modified photosynthetic viral proteins in photosynthesisWe hypothesize that viral photosynthetic peptides are integrated into the bacterial photosynthetic membranes in order to maintain photosynthesis in infected cells, that otherwise stop to photosynthesize, and that changes are introduced to the system as a whole.The proposed research will integrate concepts and techniques from metagenomics, metaproteomics and bioinformatics techniques to explore the interaction of viral PSII and PSI proteins with their host reaction center complexes, and to examine their influence on global marine photosynthesis production