Cyanobacteria of the Synechococcus and Prochlorococcus genera are important contributors to photosynthetic productivity in the open oceans1-3. Here, using pre-existing metagenomic datasets from the global ocean sampling (GOS) expedition4 as well as from viral biomes5, we show the first evidence for the presence of photosystem I (PSI) genes in genomes of marine viruses that infect these marine cyanobacteria. Recently, core photosystem II (PSII) genes were identified in cyanophages; they were proposed to be functional in photosynthesis and in increasing viral fitness by supplementing the host production of these proteins6-9. The 7 cyanobacterial core PSI genes identified in this study, psaA, B, C, D, E, K and a unique J and F fusion, form a distinctive cluster in cyanophage genomes, suggestive of selection for a distinct function in virus life cycle. The existence of this PSI cluster was confirmed with overlapping and long PCR performed on environmental DNA from the Northern Line Islands. Potentially, the 7 proteins encoded by the viral genes are sufficient to form an intact monomeric PSI complex. Projection of viral predicted peptides on the cyanobacterial PSI crystal structure10 suggested that the viral-PSI components may provide a unique way of funneling reducing power from respiratory and other electron transfer chains to PSI.

The Na ϩ -Ca 2ϩ exchanger (NCX) is a major Ca 2ϩ -regulating protein encoded by three genes: NCX1, NCX2, and NCX3. They share a sequence homology of approximately 65%. NCX1 protein is expressed ubiquitously, and NCX2 and NCX3 are expressed almost exclusively in the brain. We have shown previously ) that treatment of NCX1-transfected human embryonic kidney (HEK) 293 cells with the immunosuppressive cyclosporin A (CsA) and its nonimmunosuppressive analog PSC833 (valspodar) results in down-regulation of surface expression and transport activity of the protein without a decrease in expression of cell NCX1 protein. In this study, we show that cyclosporin A and PSC833 treatment of NCX2-and NCX3-transfected HEK 293 cells also resulted in dose-dependent down-regulation of surface expression and transport activity of the two brain NCX proteins; however, whereas CsA had no effect on total cell NCX protein expression, PSC833 reduced mRNA and cell protein expression of NCX2 and NCX3. Moreover, tacrolimus (FK506), which had no effect on NCX1 protein expression, down-regulated NCX2 and NCX3 surface expression and transport activity without any significant effect on cell protein expression. Sirolimus (rapamycin) had no effect on NCX2 and NCX3 protein expression, yet it reduced NCX2 and NCX3 transport activity. Because all of the experimental conditions in our studies were identical, presumably the different drug response is related to structural differences between NCX isoforms. Clinical studies suggested that immunosuppressive regimes of patients who have received transplants resulted in complications related to Ca 2ϩ . Expression of NCX genes is tissue-specific. Hence, our results can potentially provide a tool for choosing the immunosuppressive protocol to be used.