Sequence-independent RNA cleavages generate the primers for plus strand DNA synthesis in hepatitis B viruses: implications for other reverse transcribing elements.

Reverse transcription of RNA into duplex DNA requires accurate initiation of both minus and plus strand DNA synthesis; this in turn requires the generation of specific primer molecules. We have examined plus strand primer generation in the hepatitis B viruses, small DNA viruses that replicate via reverse transcription. The plus strand primer in these viruses is a short capped RNA derived from the 5' end of the RNA template by cleavage at a specific set of sites. To elucidate the cleavage mechanism we constructed a series of viral mutants bearing alterations in and around the cleavage sites. Our results reveal that the cleavage reaction is sequence-independent and indicate that the cleavage sites are positioned by measurement of the distance from the 5' end of the RNA. Comparison of these findings with what is known about RNase H-mediated primer generation in retroviruses and other retroid elements suggests that, despite many divergent features, some common molecular features are preserved.