Abstract
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 Mn 2+ accumulation that inhibits reverse transcription. Cytoplasmic accumulation of Mn 2+ in pmr1 cells may directly affect reverse transcriptase (RT) activity. Trace amounts of Mn 2+ potently inhibit Ty1 RT and HIV-1 RT in vitro when the preferred cation, Mg 2+, is present. Both Mn 2+ and Mg 2+ 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 Mn 2+ at this site greatly reduces catalytic activity.
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