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 retrotransposition. 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.