Nitrogen sourcing during viral infection of marine cyanobacteria

Abstract

The building blocks of a virus derived from de novo biosynthesis during infection and/or catabolism of preexisting host cell biomass, and the relative contribution of these 2 sources has important consequences for understanding viral biogeochemistry. We determined the uptake of extracellular nitrogen (N) and its biosynthetic incorporation into both virus and host proteins using an isotope-labeling proteomics approach in a model marine cyanobacterium Synechococcus WH8102 infected by a lytic cyanophage S-SM1. By supplying dissolved N as 15N postinfection, we found that proteins in progeny phage particles were composed of up to 41% extracellularly derived N, while proteins of the infected host cell showed almost no isotope incorporation, demonstrating that de novo amino acid synthesis continues during infection and contributes specifically and substantially to phage replication. The source of N for phage protein synthesis shifted over the course of infection from mostly host derived in the early stages to more medium derived later on. We show that the photosystem II reaction center proteins D1 and D2, which are auxiliary metabolic genes (AMGs) in the S-SM1 genome, are made de novo during infection in an apparently light-dependent manner. We also identified a small set of host proteins that continue to be produced during infection; the majority are homologs of AMGs in S-SM1 or other viruses, suggesting selective continuation of host protein production during infection. The continued acquisition of nutrients by the infected cell and their utilization for phage replication are significant for both evolution and biogeochemical impact of viruses.

Internal Nitrogen Pools Shape the Infection of Aureococcus anophagefferens CCMP 1984 by a Giant Virus

“…Differences in uptake of external nitrogen pools in eukaryotic algae seem to depend on the evolution the host virus pair evolved from, and not necessarily on their laboratory growth rate as previously hypothesized (Fowler and Cohen, 1948;Waldbauer et al, 2019). Both Ostreococcus tauri and A. anophagefferens have similar growth rates when grown on nitrate ( Figure 1C; Worden et al, 2004), but the virocell's reliance on the external environment for nitrogen appears to be different.…”

“…In contrast, for three marine bacterium-phage systems the majority of the phage genomes were constructed from recycled materials of host origin (Wikner et al, 1993). Moreover, the majority of the nitrogen within the cyanophage S-SM1 was derived from its host, Synechococcus WH8102, although amount varied depending on light availability (Waldbauer et al, 2019). To test this in our system, cultures acclimated to a high external NO − 3 concentration were shifted to environments with decreasing concentrations of external NO − 3 concentrations post infection.…”

“…To this end, the environment the cell is in before becoming infected is important as it shapes the physiological state of the host. Most viruses accumulate the building blocks for new progeny by some combination of degrading current host stores (i.e., the nuclear genome for nucleotides) (Agarkova et al, 2006) and driving continued uptake of nutrients from the external environment (Waldbauer et al, 2019).…”

Phage-specific metabolic reprogramming of virocells

“…Specifically, HP1 was least complementary to the host in nucleotide, amino acid and codon composition, and thus had fewer intracellular resources (nucleotides and amino acids) available for recycling than did HS2. We propose that this lower complementarity led to higher metabolic demand in the HP1 virocell and, consequently, contributed to HP1's lower relative fitness (smaller burst size), given that fitness is partially determined by a phage's ability to access and leverage resources to infect [86,87]. Because of the higher metabolic demand, HP1 virocells needed to more drastically augment translation and acquire resources, including scavenging iron and transporting sulfate into the cell to reduce it to cysteine for consumption via the glyoxylate cycle.…”

“…Globally, virocell metabolism has the potential to contribute just as much (and possibly more [84]) to ecosystem processes as the metabolism of uninfected cells in aquatic habitats [85]. Given the different resource requirements, metabolic transformations, and nutrient transport between a cell and a cell-turned-virus-factory [86,87], virocells have a unique metabolic program that influences nutrient fluxes in microbial food webs [9] and merits their study to the depths we now understand their free viral counterparts.…”