Localization by mutational analysis of transcription factor binding sequences in the U3 region of Rous sarcoma virus LTR.

The transcription factor binding sequences in the U3 region of Rous Sarcoma virus LTR have been determined by gel retardation assays using mutant synthetic oligonucleotides. The results indicate that the factor, E2BP, specifically binds to sequences TGCAATAC and TGCAACAT, which are localized between nucleotides -222 to -215 and -203 to -196, respectively. This factor is present at elevated levels in avian QT6 cells compared to mouse 3T3 and rat 2 tk- cells. E2BP binds to a sequence that is similar or identical to the sequence recognized by rat liver C/EBP. However, the two proteins are different as judged by three criteria: (i) the E2BP complex migrates slightly faster than the E2-C/EBP complex; (ii) antibodies against C/EBP neither inhibit binding of E2BP nor form a supercomplex which migrates slower than the complex formed with the factor alone; and (iii) E2BP is heat labile whereas C/EBP is heat stable. Another factor, E3BP, which binds to a sequence from -169 to -158, in the U3 region is also detected mainly in QT6 cells but not in mouse or rat cells. These results suggest that different cell-specific factors interact with different cis-acting regulatory sequences in the U3 region of RSV LTR.