Giant Viruses: Can They Make Proteins?

           Scientists from the U.S. Department of Energy Joint Genome Institute have discovered four new species of giant viruses by analyzing metagenomic data from a wastewater treatment plant sample in Klosterneuburg, Austria (1). These viruses dubbed “Klosneuviruses” possess a more complete set of translation machinery genes than any other virus known to date. Klosneuviruses, for example, contains genes for aminoacyl-tRNA (transfer ribonucleic acid) enzymes with specificity for 19 out of 20 amino acids, along with more than 20 tRNAs, an array of translation factors and tRNA modifying enzymes.
           Dr. Eugene Koonin, a co-author on the paper and an evolutionary and computational biologist at the National Institutes of Health states “since protein synthesis is one of the most prominent hallmarks of cellular life, it shows that these new viruses are more ‘cell-like’ than any virus anyone has ever seen before” (2).    
           Interestingly, in 2016 Koonin’s and Konstantin Severinov’s groups described a giant transducing Bacillus phage that relies solely on two multisubunit viral RNA polymerases (RNAPs) to transcribe its genome independently of host transcription (3). Cases where viral genome transcription is solely performed by viral RNA polymerases are extremely rare, since most viruses utilize a multisubunit RNA polymerase (RNAP) of a host organism to transcribe their genes.
           Until the discovery of the first giant virus in 2003 (4), the conventional wisdom was that viruses are not capable of making DNA, RNA, or protein synthesis themselves because they are too small and have to rely on the machinery of their host cells.  Since then, more giant viruses have been reported and extended studies challenged some of old dogmas, revealing that viruses have genetic, proteomic, and structural complexities that are comparable to those of cellular organisms.
           While viruses do not have their own ribosomes, the presence of a nearly complete set of translation components is intriguing and raises many questions. If viruses cannot make proteins without hijacking the ribosomes of host cells, then why do they need to have all of these translation-related genes? The majority of giant virus gene functions are still unknown, but perhaps one day we can confirm that viruses make proteins or peptides on their own!  

 

 References

(1) Schulz et al. Giant viruses with an expanded complement of translation system components. Science 07 Apr 2017: Vol. 356, 6333, 82-85 DOI:10.1126/science.aal4657 http://science.sciencemag.org/content/356/6333/82
(2) Discovered: Novel Group of Giant Viruses. News Release, JGI DOE, April 6, 2017 http://jgi.doe.gov/discovered-novel-group-of-giant-viruses/
(3) Lavysh et al. The genome of AR9, a giant transducing Bacillus phage encoding two multisubunit RNA polymerases. Virology 495:185 196. 10.1016/j.virol.2016.04.030. 
http://www.sciencedirect.com/science/article/pii/S0042682216300976
(4) La Scola et al. (2003). "A giant virus in amoebae". Science. 299 (5615): 2033. doi:10.1126/science.1081867. PMID 12663918