RNase H hydrolysis of the 5′ terminus of the avian sarcoma virus genome during reverse transcription

NUCLEOTIDE sequence analyses of the ends of the avian retro-virus genome 1–6 have confirmed predictions of the terminally redundant nature of the viral RNA7,8. These studies have also

provided the basis for several models of retrovirus proviral DNA synthesis, all of which have implicated the RNase H activity associated with viral reverse transcriptase in a functional role

in the continued and uninterrupted DNA transcription of the viral RNA genome1–8. According to these models DNA synthesis initiates on the tRNAtrp primer molecule located close to the 5′ end

of the viral genome9,10 and transcription proceeds to the terminus. Presumably, at this time a suitable substrate is available for the retrovirus RNase H, which is a processive

exoribonuclease requiring an unblocked terminus of the RNA moiety of RNA:DNA hybrids for activity11–13. Release of the terminally repeated nucleotides from the hybrid region of the viral

genome would create ‘sticky ends’ with the tRNAtrp-initiated DNA at the 5′ end capable of hybridising to the terminally redundant RNA genomic sequences at the 3′ end of the same or a second

35S RNA subunit. This latter reaction would facilitate uninterrupted transcription from the 5′ to the 3′ end of the viral genome resulting in genome-length DNA transcripts15–20. We have

recently obtained evidence indicating that DNA transcripts much longer than the distance between the tRNAtrp primer molecule and the 5′ end of the viral genome and containing nucleotide

sequences representing the 3′ region of the viral genome can be synthesised by the reverse transcriptase in vitro21. Thus, it seems that DNA synthesis initiated at the 5′ end of the viral

genome continues at the 3′ end in enzymatic reactions in vitro. If the avian sarcoma virus (ASV) reverse transcriptase-associated RNase H activity is required for the continued transcription

of the viral genome at the 3′ end as proposed in above models, then release of 5′ terminally-located ribonucleotides should be apparent during DNA synthesis in vitro. We report here that

ribonucleotides are indeed released from the viral RNA genome during reverse transcription, and that hydrolysis occurs at a specific site near the 5′ terminus. These studies exemplify the

first demonstration of RNase H hydrolysis occurring during reverse transcription of the retro-virus RNA genome in vitro and implicate functional role for this activity during replication of

retroviruses.

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