Evidence for splicing of avian sarcoma virus 5′-terminal genomic sequences onto viral-specific RNA in infected cells

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The 5′-terminal nucleotide sequences of the avian sarcoma virus (ASV) genome are transcribed by the reverse transcriptase in vitro into a DNA transcript that represents the entire distance (∼100 nucleotides) between the tRNATrp primer molecule and the 5′ terminus. We have used these DNA100 transcripts in hybridization reactions with ASV-specific RNA from infected avian cells and find nucleotide sequences complementary to these transcripts on all of the various size classes of viral mRNA identified. Similar hybridization results were obtained with a specific DNA transcript complementary to viral genomic nucleotide sequences between the tRNATrp primer molecule and up to, but not including, the terminal redundant sequences (DNA70), indicating that the observed hybridization of DNA100 to all size classes of viral RNA in infected cells did not reflect hybridization of DNA100 to the terminal redundant sequences at the 3′ end of the viral genome. Escherichia coli RNase H hydrolysis of RNA·DNA hybrids consisting of genomic 35S RNA obtained from virus and DNA100 transcripts indicated that viral genomic sequences complementary to these DNA transcripts were not present at sites distal to the ends of the RNA genome and therefore not adjacent to the corresponding gene sequences representing the various species of viral mRNA from infected cells. These studies suggest that the 5′-terminal genomic nucleotide sequences, or a portion thereof, are somehow added or “spliced” onto each ASV-specific mRNA species in infected cells either during or after transcription of proviral DNA for some as yet undetermined purpose.

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