Avian myeoloblastosis virus (AMV): only one side of the coin

The Avian Myeloblastsis Virus (AMV) is well known among molecular biologists because AMV stocks have been used over several decades as one of the major source of avian Reverse Transcriptase (RT). Originally identified from Rous Sarcoma Virus (RSV) [1] and Rauscher Leukaemia virus (RLV) [2], RT was also found to be an essential enzyme in the life cycle of other retroviruses and RNA viruses. The identification of RT with RNA-dependent DNA polymerase activities has enabled new developments in many areas of Biology.In the life cycle of retroviruses, RT transcribes the single stranded RNA genome into double stranded proviral DNA, which is integrated into host DNA and from which viral progeny is made. This process requires three steps, each catalyzed by the RT enzyme: synthesis of DNA from the viral RNA template, hydrolysis of the RNA strand (RNAseH activity), and syntheisis of double stranded DNA from the single stranded DNA template.Although it is known for a long time that the rate of genomic mutation is particularly high among animal retroviruses, this aspect has drawn a lot of attention in the case of HIV for which the very efficient multiplication of the virus in vivo is the source of extreme diversity [3].Mutations that are identified in the viral genome can theoretically be introduced at any of the various steps that lead to proviral DNA either during the synthesis of the minus-strand cDNA copy of the genomic RNA template, during the synthesis of the second DNA strand which are both performed by reverse transcriptase, or during the transcription of the proviral genome that is performed by RNA polymerase II. When the relative contribution of each replication stage to the retroviral mutation rate was studied with defined templates [4] it was established that mutations occur mainly at early stages, with a nearly similar mutation rate for the two polymerizations catalyzed by reverse transcriptase. However, these conclusions are weakened by the fact that the error rate