Reverse transcriptase
Naming: By convention viral polymerases are called RNA/DNA-dependent whereas cellular polymerases are called RNA/DNA-directed.
Priming:
- RNA 3'OH (Retroviridae, LTR retrotransposons, Caulimoviridae, Hepadnaviridae)
- DNA 3'OH (LINE, SINE)
- RTase at Tyr-OH residue (Caulimoviridae, Hepadnaviridae)
- a Guanosine 2'OH in retron mRNA (Retrons)
Enzymatic reaction:
- DNA nucleotidyltransferase (RNA-dependent) EC:2.7.7.49 RHEA:22508
Mechanism
Viral Rtase are characterized by a conserved motif in which two conserved aspartic acid residues are bound to two magnesium ions. The reaction begins with the deprotonation of primer's 3' OH, in a manner that doesn't require a base 
. Subsequently, the 3'-oxygen acts as a nucleophile to attack the phosphate bond and eventually releases a pyrophosphate.
Identification of four conserved motifs among the RNA-dependent polymerase encoding elements
O Poch 1, I Sauvaget, M Delarue, N Tordo
EMBO J. 1989 Dec 1;8(12):3867-74
A Structural Overview of RNA-Dependent RNA Polymerases from the Flaviviridae Family
Jiqin Wu, Weichi Liu, and Peng Gong
Int J Mol Sci. 2015 Jun; 16(6): 12943?12957