Purification and properties of the Rous sarcoma virus internal enhancer binding factor

The internal enhancer binding factor (IBF) that specifically binds sequences within the gag gene internal enhancer of Rous sarcoma virus Schmidt-Ruppin A was purified to near homogeneity from BHK cells. The polypeptides that constituted IBF DNA-binding activity were identified by sodium dodecyl sulfate-polyacrylamide gel analysis. As isolated from BHK cells, IBF consisted of two different but related polypeptides. One (IBF alpha) had a molecular weight of 40,000; the other (IBF beta) had a molecular weight of 20,000 and appeared to be a proteolytic product of IBF alpha. The site within the gag gene to which IBF bounds in vitro (internal enhancer site 2; nucleotides 856 to 878 of the Rous sarcoma virus genome) were demonstrated to function as a cis-acting transcriptional stimulatory element both in vivo and in vitro. By using HeLa cell nuclear transcription extracts, purified IBF was found to function as a trans-acting transcription factor that stimulated transcription in vitro. Purified IBF was also demonstrated to be very similar to EBP20 (K. Carlberg, T. A. Ryden, and K. Beemon, J. Virol. 62:1617-1624, 1988), and it may well belong to the same family of DNA-binding proteins.