Viruses and bacteriophages (viruses that infect bacteria) are the most abundant biological entities on our planet. In the marine environment, cyanophages (viruses infecting cyanobacteria of the genera Prochlorococcus and Synechococcus) have significant impact on ecology, evolution, and biogeochemical processes. Their self-replication is dependent on the molecular machinery of a host bacterium to generate viral progeny. Bacteriophages may have a lysogenic or lytic cycle, with the latter ultimately resulting in the lysis of the host cell. Infection by a lytic phage transforms the host bacterium into a so-called virocell. The virocell represents the intracellular state of the phage"s life cycle whose sole function is to produce virions. Phage infection induces a dramatic change in various host metabolic pathways, which is further expanded by the introduction of auxiliary metabolic genes (AMGs)1,2. Cyanophage encoded AMGs are often related to light harvesting, pigment biosynthesis and photosynthesis and are suggested to modulate and supplement the host bacterium"s metabolism to satisfy the elevated metabolic demand. Over the years, our research has focused on AMGs associated with tetrapyrrole pigment biosynthesis3,4. We have shown that phage-encoded homologs often mimic the activities of their cyanobacterial counterparts, with one notable exception: the phage's ferredoxin-dependent bilin reductase phycoerythrobilin synthase (PebS)3. This enzyme independently performs a reaction that normally requires two sequential enzymes in uninfected host cells. With the recently established recombinant phage technology5 we hope to gain insights into the role of cyanophage-encoded PebS, its importance for phage progeny production and to elucidate how cyanophage proteins interact with host metabolism.
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