Abstract
A 190-base-pair DNA-RNA hybrid containing the Moloney murine leukemia virus origin of plus-strand DNA synthesis was constructed and used as a source of template-primer for the reverse transcriptase in vitro. Synthesis was shown to initiate precisely at the known plus-strand origin. The observation that some of the origin fragments retained ribonucleotide residues on their 5' ends suggests that the primer for chain initiation is an RNA molecule left behind by RNase H during the degradation of the RNA moiety of the DNA-RNA hybrid. If the RNase H is responsible for creating the correct primer terminus, then it must possess a specific endonucleolytic activity capable of recognizing the sequence in the RNA where plus strands are initiated. The 16-base RNase A-resistant fragment which spans the plus-strand origin can also serve as a source of the specific plus-strand primer RNA. Evidence is presented that some of the plus-strand origin fragments synthesized in the endogenous reaction contain 5' ribonucleotides, suggesting that specific RNA primers for plus-strand initiation may be generated during reverse transcription in vivo as well.