A novel Met-to-Thr mutation in the YMDD motif of reverse transcriptase from feline immunodeficiency virus confers resistance to oxathiolane nucleosides

Variants of feline immunodeficiency virus (FIV) that possess a unique methionine-to-threonine mutation within the YMDD motif of reverse transcriptase (RT) were selected by culturing virus in the presence of inhibitory concentrations of (-)-β-L-2',3'-dideoxy-5-fluoro-3'-thiacytidine [(-)-FTC]. The mutants were resistant to (-)-FTC and (-)-β-L-2',3'-dideoxy- 3'-thiacytidine (3TC) and additionally exhibited low-level resistance to 2',3'-dideoxycytidine (ddC). DNA sequence analysis of the RT-encoding region of the pol gene amplified from resistant viruses consistently identified a Met-to-Thr mutation in the YMDD motif. Purified RT from the mutants was also resistant to the 5'-triphosphate forms of 3TC, (-)-FTC, and ddC. Site- directed mutants of FIV were engineered which contain either the novel Met- to-Thr mutation or the Met-to-Val mutation seen in oxathiolane nucleoside- resistant HIV-1. Both site-directed mutants displayed resistance to 3TC, thus confirming the role of these mutations in the resistance of FIV to β-L-3'- thianucleosides.