Unveiling the Hidden Life of Giant Viruses: Are They More Alive Than We Realize?

Viruses exploit host cell machinery to produce proteins, with certain large viruses encoding essential components within their genomes to instruct host cells to generate viral proteins. This phenomenon emphasizes how giant viruses challenge the distinction between living and nonliving entities.

Since the discovery of the mimivirus in Bradford, England in 2003, which infects amoebas, biologists have increasingly focused on these giant viruses. Some exhibit sizes larger than typical bacteria, complex shapes, and possess numerous genes.

Among these genes are those that code for components involved in translation—the biological process that turns genetic information into proteins. In cellular biology, translation occurs through ribosomes, initiated by molecular assemblies known as initiation complexes.

To investigate whether giant viruses possess a similar system, Max Fells and his team from Harvard Medical School explored the dynamics within infected amoebas and the manipulations by mimivirus post-infection.

The researchers isolated ribosomes from infected cells and identified the viral proteins linked to them. “This was our initial clue that these might be the elements we were seeking,” said Fells.

Subsequently, they knocked out the gene responsible for the viral complex by substituting it with a modified DNA sequence, resulting in a virus that could not synthesize the corresponding protein. This intervention decreased virus production by up to 100,000-fold and severely inhibited the formation of new infectious particles.

These findings collectively indicate that during an infection, viral complexes potentially redirect the protein synthesis machinery of the host to significantly boost the production of viral structural proteins, even under extreme conditions like nutrient scarcity and oxidative stress, which typically hinder protein synthesis in host cells.

This discovery introduces a profound evolutionary inquiry: how did these viruses acquire such capabilities? Some researchers propose that giant viruses may descend from ancient cellular life forms, while others suggest they evolved from typical viruses through gene acquisition from their hosts.

“Giant viruses have acquired a diverse array of cellular machinery from their eukaryotic hosts over evolutionary time,” stated Frank Aylward from Virginia Tech, who was not part of the study. Genetic exchange can occur during viral infection, allowing natural selection to favor advantageous genes over extended evolutionary periods.

Many of the largest viruses dominate the internal environment of single-celled organisms, which presents more variability than the relatively stable environments of multicellular hosts. Consequently, this flexible control over protein synthesis may confer a significant evolutionary advantage, Aylward noted.

This research also raises critical questions. The mimivirus genome comprises approximately 1,000 proteins, the majority of which remain functionally enigmatic. It remains unclear how these viruses intricately control protein production throughout a single infection cycle.

“Viruses have traditionally been regarded as passive participants in the evolution of living systems,” stated Hiroyuki Ogata from Kyoto University, Japan. “This study demonstrates that giant viruses can reconfigure molecular systems that are fundamental across the spectrum of life.”