Mutations in the ribonuclease H active site of HIV-RT reveal a role for this site in stabilizing enzyme-primer-template binding

The RNase H activity of HIV-RT is coordinated by a catalytic triad (E478, D443, D498) of acidic residues that bind divalent cations, We examined the effect of RNase H deficient E-478-->Q and D-549-->N mutations that do not alter polymerase activity on binding of enzyme to various nucleic acid substrates. Binding of the mutant and wild-type enzymes to various nucleic acid substrates was examined by determining dissociation rate constants (k(off)) by titrating both Mg2+ and salt concentrations. In agreement with the unaltered polymerase activity of the mutant, the k(off) values for the wild-type and mutant enzymes were essentially identical using DNA-DNA templates in the presence of 6 MM Mg2+. However, with lower concentrations of Mg2+ and in the absence of Mg2+, although both enzymes dissociated more rapidly, the mutant enzymes dissociated several-fold more slowly than the wild type. This was also observed on RNA-DNA templates. These results indicate that alterations in residues essential for Mg2+ binding have a pronounced positive effect on enzyme-template stability and that the negative residues in the RNase H region of the enzyme have a negative influence on binding in the absence of Mg2+. In this regard RT is similar to other nucleic acid cleaving enzymes that show enhanced binding upon mutation of active site residues.