Viruses mutate and trade genes all the time, which serves them really well but has not been so great for us. In addition to making it that much trickier for our immune systems to combat them, it has also made it much more difficult to study their evolution and to categorize them taxonomically. As we continue to figure them out, researchers at Portland State University in Oregon have discovered a completely new class of virus: an RNA-DNA hybrid virus.
Analyzing samples from the lake using metagenomic techniques, the scientists found a circular, single-stranded DNA virus with a major capsid protein similar to those only seen thus far in single-stranded RNA viruses. After the initial identification, they went back and isolated and sequenced the complete genome of this new virus from a clean sample. When they searched for similar candidate genomes in databases from the Global Ocean Survey, they found three related matches.
To date, viruses have come in three distinct flavors: RNA-only; DNA-based; and retroviruses, which have RNA genomes that must be reverse transcribed into DNA at some point during their life cycle. Exchange of genes has been rampant within each class, but this is the first clear example of the lateral transfer of genes from an RNA-only to a DNA-only virus. The authors do not know how, or when, this event might have happened.
“The provenance of this particular virus genome is probably recent, because of the relatively low levels of sequence divergence between the homologous viral proteins,” said Stedman. “But the idea of putting genes from an RNA virus into a DNA virus – the fact that this can happen – means that it could have also happened at any time in the past, and probably did.”
In a nod to the virus’ unusual habitat, Diemer presented his findings at NASA’s Astrobiology Conference in Atlanta, Georgia, on April 17. He suggests that determining the mechanism responsible for this direct recombination between RNA and DNA viruses may be an important step in understanding the transition from the ancient RNA world to the DNA world we now inhabit.
- Diemer, G.S. and K.M. Stedman. A novel virus genome discovered in an extreme environment suggests recombination between unrelated groups of RNA and DNA viruses. Biology Direct 2012, 7:13.