Giant Viruses—Big Surprises

Abstract

Viruses are the most prevalent infectious agents, populating almost every ecosystem on earth. Most viruses carry only a handful of genes supporting their replication and the production of capsids. It came as a great surprise in 2003 when the first giant virus was discovered and found to have a >1 Mbp genome encoding almost a thousand proteins. Following this first discovery, dozens of giant virus strains across several viral families have been reported. Here, we provide an updated quantitative and qualitative view on giant viruses and elaborate on their shared and variable features. We review the complexity of giant viral proteomes, which include functions traditionally associated only with cellular organisms. These unprecedented functions include components of the translation machinery, DNA maintenance, and metabolic enzymes. We discuss the possible underlying evolutionary processes and mechanisms that might have shaped the diversity of giant viruses and their genomes, highlighting their remarkable capacity to hijack genes and genomic sequences from their hosts and environments. This leads us to examine prominent theories regarding the origin of giant viruses. Finally, we present the emerging ecological view of giant viruses, found across widespread habitats and ecological systems, with respect to the environment and human health.

Biochemical analysis of a sugar 4,6‐dehydratase from Acanthamoeba polyphaga Mimivirus

“…And the researchers who were simply curious about proteins from extreme thermophiles discovered the “ Taq ” and “ Pfu ” polymerases that revolutionized the use of PCR in basic research as well as in clinical and forensics settings . In 2003, the curiosity‐driven discovery of the giant viruses that infect amoeba forever changed the field of virology by challenging the very definition of a virus and of life in general …”

“…They typically infect amoeba ( A . polyphaga ) and have been isolated from both natural aquatic settings as well as from manmade pools and cooling wastewater systems . Strikingly, these viruses are larger than some archaeal or bacterial cells and are visible under the light microscope.…”

“…Indeed, the Gram‐positive staining of APMV led to its misidentification in 1992 as a small bacterium . Whereas ~69% of viruses identified thus far have genomes encoding for less than 10 proteins, the >1 Mbp genome of APMV is predicted to encode 979 proteins . Yet, despite the discovery of APMV in 2003, many of its DNA‐encoded proteins remain largely uncharacterized .…”

In-Gel Isolation and Characterization of Large (and Other) Phages

“…The more rapidly one can (1) obtain plaques of environmental phages and (2) characterize phage particles in a plaque, the more effective is screening of newly isolated phages for several purposes. These purposes include phage therapy of infectious disease [8][9][10][11] and genomic sequence-analysis of the evolution of both phages and their hosts [12][13][14]. Possibly, phage evolution is linked, via horizontal gene transfer, to eukaryotic evolution [15][16][17][18].…”

Boiling Acid Mimics Intracellular Giant Virus Genome Release

“…By contrast, some of the smallest viruses include the porcine circovuris (17 nm capsid, ~2000 base genome, four proteins, (Dhindwal et al, 2019)) and the human rhinovirus (~7200 base genome, 30 nm capsid, 11 proteins, (Jacobs et al, 2013)). ~69% of known viruses encode for less than 10 proteins (Brandes and Linial, 2019), highlighting the complexity of GV and the true extent of our lack of knowledge concerning this new class of viruses.…”