Insights into DNA polymerization mechanisms from structure and function analysis of HIV-1 reverse transcriptase.

When the single-stranded RNA genome of HIV-1 is copied into double-stranded DNA, the viral enzyme reverse transcriptase (RT) catalyzes the addition of approximately 20,000 nucleotides; however, the precise mechanism of nucleotide addition is unknown. In this study, we attempt to integrate the genetic data and biochemical mechanism of DNA polymerization with the structure of HIV-1 RT complexed with a dsDNA template-primer. The first step of polymerization involves the physical association of a polymerase with its nucleic acid substrate. A comparison of the structures of HIV-1 RT in the presence and absence of DNA indicates that the tip of the p66 thumb moves approximately 30 A upon DNA binding. This conformational change permits numerous interactions between residues of alpha-helices H and I in the thumb subdomain and the DNA. Measurements of DNA binding affinity for nucleic acids with double-stranded DNAs that have an increasing number of bases in the template overhang and molecular modeling suggest that portions of beta 3 and beta 4 within the fingers subdomain bind single-stranded regions of the template. Measurements of nucleotide incorporation efficiency (kcat/Km) show that the binding and incorporation of the next complementary nucleotide are not dependent on the length of the template overhang. Molecular modeling of an incoming nucleotide triphosphate (dTTP), based in part on the position of mercury atoms in a RT/DNA/Hg-UTP/Fab structure, suggests that portions of secondary structural elements alpha C-beta 6, alpha E, beta 11b, and beta 9-beta 10 determine the topology of the dNTP-binding site. These results also suggest that nucleotide incorporation is accompanied by a protein conformational change that positions the dNTP for nucleophilic attack. Nucleophilic attack by the oxygen atom of the 3'-OH group of the primer strand could be metal-mediated, and Asp185 may be directly involved in stabilizing the transition state. The translocation step may be characterized by rotational as well as translational motions of HIV-1 RT relative to the DNA double helix. Some of the energy required for translocation could be provided by dNTP hydrolysis and could be coupled with conformational changes within the nucleic acid. A structural comparison of HIV-1 RT, Klenow fragment, and T7 RNA polymerase identified regions within T7 RNA polymerase which are not present in the other two polymerases that might help this polymerase to remain bound with nucleic acids and contribute to the ability of the T7 RNA polymerase to polymerize processively.