What is the role of cyanophage-encoded auxiliary metabolic genes in the virocell?

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Marine Cyanobacteria of the genera Synechococcales and Prochlorococcales are the most abundant prokaryotic phototrophs in the world’s ocean. These Cyanobacteria are highly challenged by viral infection with up to 40 % of the bacterial population being infected by phages. In the marine environment, cyanophages have significant impact on ecology, evolution and biogeochemical processes. They are involved in nutrient recycling, host population control and serve as vehicles for gene transfer and niche adaptation. Infection of a bacterium by a phage transforms the host cell into a so-called virocell where both viral and host genomes are expressed in parallel. The infected host cell can no longer divide, its sole function being to produce virions to propagate phage genes. In this state, significant changes in diverse host metabolic pathways are induced. Cyanophages furthermore expand the virocell metabolism by introducing auxiliary metabolic genes (AMGs) often related to photosynthesis and light-harvesting to supplement the required metabolic demand. A fundamental understanding of the biochemical characteristics of the AMG encoded proteins is required to elucidate the viral strategies that are essential for an optimal viral replication cycle. Biochemical characterization of phage encoded AMGs related to light-harvesting and pigment biosynthesis indicate that they often encode highly efficient enzymes that can differ considerably from their host counterparts, likely enabling the redirection of the virocell metabolism. Within this priority program, the role of AMG encoded proteins in the virocell, their interaction with host metabolic proteins as well as their dispensability will be investigated. Specifically, the virocell of Synechococcus sp. WH8109 challenged by the cyanophages Syn9 and S-SSM7 under blue and green light representing different depths in the water column, will be explored. Using mass spectrometry-based proteomics the ratio of host to phage proteins with emphasis on those related to light-harvesting and pigment biosynthesis will be determined. Both strategies aim at gaining insight into the role of AMGs during infection and how they shape the virocell metabolism. Genetic manipulation of the phage genome will provide a better understanding of the essential nature of AMGs on virion replication. Additionally, physiological parameters related to photosynthesis (light saturation points, O2 release, pH), as well as ecological parameters such as nutrient release (protein, glycogen, phosphates), will be assessed in host and virocells. Finally, given the strong expertise in protein biochemistry, selected AMGs will be investigated on the molecular level and their biochemical functions compared to that of the host counterpart. Overall, this project will add to the global knowledge base on the role of cyanophages in regulating oceanic photosynthesis and nutrient cycling.