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

Abstract

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 RNA^7,8. 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 genome^1–8. According to these models DNA synthesis initiates on the tRNA^trp primer molecule located close to the 5′ end of the viral genome^9,10 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 activity^11–13. Release of the terminally repeated nucleotides from the hybrid region of the viral genome would create ‘sticky ends’ with the tRNA^trp-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 transcripts^15–20. We have recently obtained evidence indicating that DNA transcripts much longer than the distance between the tRNA^trp 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.