The discovery of the genes psbA and psbD, encoding the D1 and D2 core components of the photosynthetic reaction center PSII (photosystem II), in the genome of the bacteriophage S-PM2 (a cyanomyovirus) that infects marine cyanobacteria begs the question as to how these genes were acquired. In an attempt to answer this question, it was established that the occurrence of the genes is widespread among marine cyanomyovirus isolates and may even extend to podoviruses. The phage psbA genes fall into a clade that includes the psbA genes from their potential Synechococcus and Prochlorococcus hosts, and thus, this phylogenetic analysis provides evidence to support the idea of the acquisition of these genes by horizontal gene transfer from their cyanobacterial hosts. However, the phage psbA genes form distinct subclades within this lineage, which suggests that their acquisition was not very recent. The psbA genes of two phages contain identical 212-bp insertions that exhibit all of the canonical structural features of a group I self-splicing intron. The different patterns of genetic organization of the psbAD region are consistent with the idea that the psbA and psbD genes were acquired more than once by cyanomyoviruses and that their horizontal transfer between phages via a common phage gene pool, as part of mobile genetic modules, may be a continuing process. In addition, genes were discovered encoding a high-light inducible protein and a putative key enzyme of dark metabolism, transaldolase, extending the areas of host-cell metabolism that may be affected by phage infection. Strains of unicellular cyanobacteria Synechococcus and Prochlorococcus are abundant in the world's oceans, and they dominate the prokaryotic component of the picophytoplankton. Together, they contribute 32-89% of primary production in oligotrophic regions of the oceans (1-4). The recent discovery (5) that a phage infecting marine Synechococcus strains encodes key photosynthetic genes has important implications for our understanding of the effect of phage infection on the photosynthetic picophytoplankton physiology and consequent impact on major biogeochemical cycles. The acquisition of photosynthesis genes by a phage, presumably by horizontal gene transfer, begs the questions of (i) how common the possession of photosynthesis genes by such phages is, (ii) where the acquired genes were from, and (iii) whether the acquisition was a single rare ancestral event or a common phenomenon in the oceans.The cyanobacterial picoplankton, together with all organisms capable of oxygenic photosynthesis, possess two photosynthetic reaction centers, PSI and PSII (photosystems I and II). The PSII complex is a large protein-pigment assembly in the thylakoid membrane that catalyses the light-dependent oxidation of water to molecular oxygen. At the core of PSII lies a heterodimer of two related proteins, D1 and D2, which binds the pigments and cofactors necessary for primary photochemistry. During active photosynthesis, D1 and, to a lesser extent, D2 turn over rapidly as...
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