The U3 region of the long terminal repeat of a subgroup A transformation-defective rous sarcoma virus (tdPH2010) converts a noncytopathic virus to a cytopathic virus.

The molecular basis of the cytopathic effects (CPE) of the transformation-defective avian retrovirus mutant, tdPH2010, was studied. tdPH2010 is a subgroup A virus isolated from the Schmidt-Ruppin (NY) subgroup A strain (SRA(NY)). Subgroup A avian retroviruses are generally considered noncytopathic. Integrated tdPH2010 was molecularly cloned from infected quail cells. A noncytopathic, transformation-defective control strain, BSU, was created by deleting the src gene from the molecularly cloned wild type SRA(NY) virus. Chimeras between tdPH2010 and BSU were constructed and viruses were recovered from transfected chick embryo fibroblasts. Growth curves of cells infected with chimeric viruses indicated that the long terminal repeat (LTR) of tdPH2010 converts BSU to a cytopathic virus. Nucleotide sequencing revealed two point mutations unique to tdPH2010 in the U3 region of LTR at positions -126 and -23 from the transcription start site. Both mutations were located inside or near the promoter/enhancer elements of U3. The mutation at -126 (G to T) converted one of the very well-conserved pentanucleotide repeat (PRE) motifs from GGTGG to GGTGT. The other at -23 (G to A) is located next to the TATA box. The G at this position is conserved in all other known avian retrovirus promoters.