Structure, function, and evolution of bacterial reverse transcriptase.

The discovery of retroelements in the prokaryotes raises intriguing questions concerning their roles in bacteria and the origin and evolution of reverse transcriptases. We first discuss a possible structure of bacterial reverse transcriptases on the basis of the known three-dimensional structure of HIV-1 reverse transcriptase, and how such a putative three-dimensional structure is able to recognize a single primer-template RNA molecule to initiate DNA chain elongation from the 2'-OH group of an internal G residue. This reaction leads to the production of a unique RNA-DNA complex called msDNA (multicopy single-stranded DNA) in which a single-stranded DNA branches out from an RNA molecule via a 2',5'-phosphodiester linkage. Second, the mobility of the bacterial retroelements called retrons, responsible for the production of msDNA, are discussed and compared with the mobility of group I and group II introns. Third, the original and evolution of bacterial reverse transcriptases are discussed in light of the question of whether the bacterial reverse transcriptases are older than eukaryotic reverse transcriptases.