Two DNA Polymerases: HIV Reverse Transcriptase and the Klenow Fragment of Escherichia coli DNA Polymerase I

The three-dimensional structures of two DNA polymerases are presently known (Ollis et al. 1985; Kohlstaedt et al. 1992). One, the Klenow fragment (KF) of E. coli polymerase I, uses a DNA template whereas the other, HIV reverse transcriptase (RT), can employ either RNA or DNA as a template. As with all DNA polymerases, a primer is required for the initiation of DNA synthesis (for a detailed discussion of polymerases, see Kornberg and Baker 1992). KF initiates DNA synthesis at nicks and gaps in DNA and extends the DNA primer strand on the DNA template. In the case of RT, the first round of DNA synthesis is initiated by a tRNA primer bound to the viral RNA template. This enzyme must be able to recognize tRNA, partially unwind its structure, and facilitate the hydrogen-bonding interaction between the tRNA and the template.