The move from RNA to DNA and the necessity of reverse transcriptase

Question

In this piece about giant viruses and the origin of life, in the context of the move from RNA world to DNA world, virologist Valerian Dolja states:

In order to move from RNA to DNA, you need an enzyme called reverse transcriptase

Is this true though? Don't many DNA polymerases work with both, ssDNA and ssRNA templates, i.e. they function as reverse transcriptases although they're not "true" reverse transcriptases. So, in order to move from RNA to DNA, a reverse transcriptase was not a necessity like ribonucleotide reductase was for RNA to Uracil-DNA and thymidylate synthase was for Uracil-DNA to DNA?

Answer 1

Yes, this is true. This flow of information is called the "Central dogma of Molecular Biology". It basically describes the flow of information from DNA to RNA to proteins. Since a virus needs to amplify its genetic information before it can take over the infected cell and make only viral proteins and its genetic information, it has two possible options.

DNA viruses can use the cells replication and transcription machinery to make copies and express proteins. RNA viruses either have to use a reverse transcriptase to make DNA from its RNA genome to go on like the DNA viruses. Or they have to have a special, RNA-dependent (works only on RNA templates) RNA-polymerase (our RNA polymerase are all DNA-dependent and work only on DNA templates) to make more copies of their genome. Our DNA polymerases are dependent on DNA, the same is true for our RNA polymerases. So if the virus is not encoding (and actually bringing a few molecules with it in the virus) of the RT-polymerase or the special RNA-dependent RNA polymerase it will not be able to spread.

Using ribonucleotide reductases would theoretically be possible, but it is not the way nature went. And since a cell contains huge amounts of RNA, I doubt this would be a feasible way, as the enzyme would reduce all RNA. And later on the new viruses also needs new (not reduced RNA) to be packaged into new viruses. This would also be problematic.

Answer 2

I don't know if this is helpful in the context of moving from RNA to DNA world, but I do know that certain mutations to T7 RNA polymerase will allow that enzyme to use 2' deoxy nucleotides in addition to the normal ribonucleotides. These mutations were discovered by humans engineering the enzyme, but it's feasible that some transitional enzyme could have utilized both types of nucleotide. Then, those individuals with more DNA were favored because the DNA backbone is more stable than RNA backbone, eventually selecting for the modern DNA to RNA to Protein model.

Here is an article: http://www.sciencedirect.com/science/article/pii/S0168165613002940