Pyridoxal-5′-Phosphate Inhibits the Polymerase Activity of a Recombinant RNase H-Deficient Mutant of HIV-1 Reverse Transcriptase

We have investigated the ability of pyridoxal-5′-phosphate to inhibit a recombinant deletion mutant of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) which is missing the last 23 amino acids of the C-terminus. This mutant reverse transcriptase is characterized by normal polymerase activity as compared with full-length enzyme; however, it has no RNase H activity. Inhibition studies with pyridoxal-5′-phosphate showed several differences as compared with inhibition of full-length enzyme: (1) Inhibition of mutant reverse transcriptase was independent of divalent cation, (2) Either substrate alone could protect mutant reverse transcriptase from inactivation by pyridoxal-5′-phosphate, and (3) stoichiometry of pyridoxal-5′-phosphate binding to mutant reverse transcriptase was 2 mol/mol under the same conditions in which 1 mol/mol bound to full-length enzyme. Furthermore, in the presence of either substrate alone, the stoichiometry of pyridoxal-5′phosphate binding to the mutant was reduced to 1 mol/mol. These results indicate that the second binding site for pyridoxal-5′-phosphate seen in the mutant reverse transcriptase is at or near the primer-template binding site of the enzyme. They also suggest that the RNase H domain of HIV RT plays a functional role in substrate binding at the polymerase domain.