A new study reveals that giant viruses, like the mimivirus, encode parts of the cellular protein-making machinery, allowing them to direct their amoeba hosts more effectively. This capability blurs the line between living and non-living entities. Researchers suggest it enhances viral production even under stressful conditions.
Giant viruses have intrigued biologists since 2003, when a mimivirus was identified in a water sample from Bradford, UK. This virus, which infects amoebae, is larger than many bacteria and features intricate structures along with hundreds of genes.
Typically, viruses depend on host cells to produce proteins, but some giant viruses incorporate elements of the translation machinery—the process that converts genetic information into proteins—directly into their genomes. Translation involves ribosomes and initiation complexes in cells.
Max Fels at Harvard Medical School and colleagues investigated this in infected amoebae. They isolated ribosomes from these cells and found associated viral proteins. “That was the first hint that they could be the factors we were looking for,” says Fels.
To test the role of these viral proteins, the team modified the virus's genes to prevent their production. This reduced viral output by up to 100,000-fold and severely limited new infectious particle formation.
The findings indicate that the viral complex redirects the host's protein-synthesis system toward producing viral proteins, functioning even during nutrient shortages or oxidative stress, which normally hinder host protein synthesis.
This raises evolutionary questions: did giant viruses evolve from ancient cells, or did they acquire genes from hosts? “Giant viruses have acquired a wide range of cellular machinery from their eukaryotic hosts throughout their evolution,” notes Frank Aylward at Virginia Tech, who was not involved in the study. Gene transfer during infections, followed by natural selection, likely preserved beneficial genes.
Such viruses target single-celled hosts like amoebae, where environments vary more than in multicellular organisms, making adaptable protein control advantageous.
The mimivirus genome codes for about 1,000 proteins, but most functions remain unknown, including precise regulation during infection cycles. “Viruses have long been considered rather passive entities in the evolution of living systems,” says Hiroyuki Ogata at Kyoto University in Japan. “This study shows that giant viruses can reshape molecular systems that are otherwise stably conserved across the domains of life.”
The research appears in Cell (DOI: 10.1016/j.cell.2026.01.008).
