Analysis of the polymerization kinetics of homodimeric EIAV p51/51 reverse transcriptase implies the formation of a polymerase active site identical to heterodimeric EIAV p66/51 reverse transcriptase

Homodimeric EIAV p51/51 and heterodimeric EIAV p66/51 reverse transcriptase were purified in order to compare the different modes of DNA synthesis supported by the enzymes. Analysis of the dimerization behavior of the EIAV enzymes indicates that the dimer stability of EIAV reverse transcriptase enzymes is higher than that of their HIV-1 reverse transcriptase counterparts. EIAV p51/51 polymerizes DNA distributively whereas DNA synthesis by EIAV p66/51 is processive. Steady-state and pre- steady-state kinetic analyses of primer/template binding and nucleotide incorporation were performed with both enzymes to determine the reasons for the different polymerization behavior. Equilibrium fluorescence titrations demonstrated that the K(d) values of EIAV p51/51 for binding of DNA/DNA and DNA/RNA substrates are increased 10-fold and 28-fold, respectively, as compared to EIAV p66/51. Stopped-flow measurements with DNA/DNA show that the increase in the K(d) is in part due to a 17.4-fold higher dissociation rate constant (k_-1) for EIAV p51/51. Additionally, with EIAV p51/51, k(diss) is increased 7-fold for DNA/DNA and 14-fold for DNA/RNA primer/template substrates, respectively. The lack of the RNase H domain in EIAV p51/51 leads to differences in the pre-steady-state kinetics of nucleotide incorporation on DNA/DNA and DNA/RNA templates. The burst of both enzymes is composed of two phases for both substrates, and the values for the corresponding pre- steady-state burst rates, k(pol1) and k(pol2), are similar for both enzymes, implying the formation of identical polymerase active sites. However, the amplitudes of the two phases differ with DNA/DNA templates, indicating a different distribution between two states varying greatly in their kinetic competence.