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Reverse Transcription of a Naturally Occurring Nonretroviral RNA Produces a Precise Deletion in the Majority of Its cDNA Products

A precise, reproducible deletion made during in vitro reverse transcription of RNA2 from the icosahedral positive‐stranded Helicoverpa armigera stunt virus (Tetraviridae) is described. The deletion, located between two hexamer repeats, is a 50‐base sequence that includes one copy of the hexamer repeat. Only the Moloney murine leukemia virus reverse transcriptase and its derivative Superscript I, carrying a deletion of the carboxy‐terminal RNase H region, showed this response, indicating a template‐switching mechanism different from one proposed that involves a RNase H? dependent strand transfer. Superscript II, however, which carries point mutations to reduce RNase H activity, does not cause a deletion. A possible mechanism involves the enzyme pausing at the 3′ side of a stem‐loop structure and the 3′ end of the nascent DNA strand separating from the template and reannealing to the upstream hexamer repeat.