The fidelity of reverse transcription differs in reactions primed with RNA versus DNA primers.

Reverse transcriptase enzymes (RT) convert single-stranded retroviral RNA genomes into double-stranded DNA. The RT enzyme can use both RNA and DNA primers, the former being used exclusively during initiation of minus- and plus-strand synthesis. Initiation of minus-strand DNA synthesis occurs by extension of a tRNA primer that is associated with the viral genome, and plus-strand DNA synthesis is initiated from an RNase H- resistant polypurine tract of the genomic RNA that remains bound to the newly synthesized minus-strand DNA. All other phases of reverse transcription represent elongation of a DNA primer. We demonstrate that the polymerase fidelity of RT enzymes is significantly higher in tRNA-primed reverse transcription compared with DNA-primed reactions. Two mechanistic explanations can be proposed. First, the type of template-primer (T- P) duplex (RNA-RNA versus RNA-DNA) may affect the RT enzyme conformation such that the discrimination against incorrect nucleotides is affected. Second, the tRNA primer may act as a fidelity co-factor through specific association with the RT enzyme. According to the latter hypothesis, the increased fidelity observed for an RNA-RNA T-P should persist at a distance from the initiation site, where the enzyme-bound nucleic acid duplex will consist of RNA-cDNA. However, we measured that the effect of tRNA on the fidelity is detectable only at a short distance from the initiation site. These results indicate that the type of T-P duplex influences the fidelity of reverse transcription, suggesting that two small segments of the viral genome downstream of the initiation sites for minus- and plus-strand DNA synthesis are copied with a fidelity that is greater than average.