Inhibition of reverse transcription in vivo by elevated manganese ion concentration

Mutations in PMR1, a yeast gene encoding a calcium/manganese exporter, dramatically decrease Ty1 retro-transposition. Ty1 cDNA is reduced in pmr1 mutant cells, despite normal levels of Ty1 RNA and proteins. The transposition defect results from Mn²⁺ accumulation that inhibits reverse transcription. Cytoplasmic accumulation of Mn²⁺ in pmr1 cells may directly affect reverse transcriptase (RT) activity. Trace amounts of Mn²⁺ potently inhibit Ty1 RT and HIV-1 RT in vitro when the preferred cation, Mg²⁺, is present. Both Mn²⁺ and Mg²⁺ alone activate Ty1 RT cooperatively with Hill coefficients of 2, providing kinetic evidence for a dual divalent cation requirement at the RT active site. We propose that occupancy of the B site is the major determinant of catalytic activity and that Mn²⁺ at this site greatly reduces catalytic activity.