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Saturday, June 9, 2012

Recombination between RNA and DNA viruses



Viruses are classified into three broad groups depending on their nucleic acid content. There are RNA viruses, DNA viruses, and retroviruses (that contain RNA but use a DNA intermediate in order to replicate). Examples of these three types are rhinoviruses, parvoviruses and HIV, respectively. Although genetic material is readily exchanged within groups, gene transfer from one group to another has not been observed.  Until now.

Geoffrey Deimer and Kenneth Stedman from Portland University discovered a virus with a genome consisting of a circular strand of DNA. So far, this isn’t unusual. However, this particular virus includes the gene for a protein that has only been seen in RNA viruses. This strongly suggests that this gene hopped from an RNA virus into a DNA virus, an unprecedented event.

Think for a minute what this must have entailed. The DNA within our cells resides in the nucleus. It gets transcribed into RNA and that RNA migrates into the cytoplasm where it is translated into proteins. The RNA virus skips this DNA step. Once an RNA virus enters a cell, the machinery in that cell’s cytoplasm translates the genes directly into protein. The RNA virus can ignore the cell nucleus as no DNA is involved at all. To be incorporated into a DNA genome, the viral RNA gene would have had to have been reverse transcribed into DNA, requiring an enzyme that retroviruses supply, but that is not present in cells or other types of viruses. Yet, somehow this event must have occurred to create the virus found by the researchers.

Exchanging bits between two different RNA viruses or between two DNA viruses requires little more than a bit of cutting and pasting. If viruses can also transfer genes between groups, the possibilities for new combinations go up dramatically.