A POSSIBLE ROLE FOR CYSTEINE RESIDUES IN THE FIDELITY OF DNA-SYNTHESIS EXHIBITED BY THE REVERSE TRANSCRIPTASES OF HUMAN IMMUNODEFICIENCY VIRUSES TYPE-1 AND TYPE-2

HIV reverse transcriptases (RTs) have few cysteine residues relative to other RTs and retain their DNA polymerization functions following chemical modification by thiol-specific reagents. The functional role of the cysteines in the fidelity of the DNA-dependent DNA synthesis of HIV RTs has been addressed by chemical modification of the wild-type enzymes in combination with the analysis of an enzymatically active mutant HIV-1 RT in which all cysteines were modified to serines. We have observed an increase in 3'-terminal mispair extension efficiency exhibited by chemically modified HIV-1 and HIV-2 RTs. The possible involvement of cysteine residues was further substantiated using the cysteine-free mutant HIV-1 RT that displays an increased efficiency of mispair extension. These results provide evidence for a possible role of cysteine residues in the fidelity of DNA synthesis catalyzed by HIV RTs.