Two point mutations in the U3 region of the long terminal repeat convert a subgroup A transformation-defective Rous sarcoma virus to a cytopathic virus

To elucidate the mechanism of cytopathicity of the transformation-defective avian retrovirus tdPH2010, we examined the function of two point mutations we had previously found in the long terminal repeat U3 region of this virus. Our previous studies showed that the U3 region was responsible for the cytopathic effects. These mutations were a G-to-T mutation at position -126 from the transcription start site and a G-to-A mutation at -23, Site-directed mutagenesis was performed on a noncytopathic, wild-type virus BSU to alter the nucleotides at these two positions, one at a time, to those of tdPH2010. Cell growth assay using the altered viruses revealed that host cell growth was retarded only when both of these mutations were present. The two additional mutations previously found in the direct repeat l-polypurine tract (DR1-PPT) region of tdPH2010 were present also in the noncytopathic strain tdPH2013, Site-directed mutagenesis confirmed that these two mutations had indeed no role in the cytopathic effect of tdPH2010. None of these mutations influenced virus production from the infected cells. We conclude that the cytopathic effects by tdPH2010 are ascribed to the two point mutations in the U3 region.