Physiological Magnesium Concentrations Increase Fidelity of Diverse Reverse Transcriptases from HIV-1, HIV-2, and Foamy Virus, but not MuLV or AMV

Abstract

Reverse transcriptases (RTs) are typically assayed in vitro using optimized Mg2+ concentrations (~ 5-10 mM) that are several-fold higher than physiological cellular free Mg2+ (~ 0.5 mM). Analysis of fidelity using lacZα-based α-complementation assays showed that tested HIV RTs, including HIV-1 from subtype B (HXB2-derived), HIV-2, subtype A/E, and several drug-resistant HXB2 derivatives all showed significantly higher fidelity using physiological Mg2+. This also occurred with prototype foamy virus (PFV) RT. In contrast, Moloney murine leukemia virus (MuLV) and avian myeloblastosis virus (AMV) RTs demonstrated equivalent fidelity in both low and high Mg2+. In 0.5 mM Mg2+, all RTs demonstrated ≍ equal fidelity, except for PFV RT which showed higher fidelity. A Next Generation Sequencing (NGS) approach that used barcoding to accurately determine mutation rates and profiles was used to examine the types of mutations made by HIV-1 (subtype B, wild type) in low (0.5 mM) and high (6 mM) Mg2+ with DNA or RNA that coded for lacZα. Unlike the α-complementation assay, which is dependent on LacZα activity, the NGS assay scores mutations at all positions and of every type. A ~ 4-fold increase in substitution mutations was observed in high Mg2+. The general trend was an exacerbation in high Mg2+ of more common mutation in low Mg2+, rather than the creation of new mutation hotspots. These findings help explain why HIV RT displays lower fidelity in vitro (with high Mg2+ concentrations) than other RTs (e.g., MuLV and AMV), yet cellular fidelity for these viruses is comparable.