Viral sequences determining the oncogenicity of avian leukosis viruses

Our analyses of the sequence of the recombinant virus NTRE7 and comparison with its endogenous and exogenous ALV parents indicate that a section of approximately 400 base pairs at the 3' end of the genome encodes a function which is required for oncogenicity. This section includes an exogenous virus specific region (XSR) of approximately 148 base pairs and the U3 region of the LTR. At present it seems most likely that the function required for oncogenicity is the transcription promoter contained in the U3 region. Assays designed to measure promoter activity of endogenous and exogenous LTRs show a 10 fold difference between the two. This result is consistent with the notion that RAV-0 is non-oncogenic because its promoter activity falls below the limit required for oncogene activation. Our findings suggest that there may be a critical threshold for oncogene activation for which the RAV-2 value represents an upper limit.