CONFORMATIONAL ALTERATION OF OCT-1 UPON DNA-BINDING DICTATES SELECTIVITY IN DIFFERENTIAL INTERACTIONS WITH RELATED TRANSCRIPTIONAL COACTIVATORS

VP16 (termed VP16-H here) of herpes simplex virus (HSV) belongs to a f amily of related regulatory proteins which includes VP16-B of bovine h erpesvirus (BHV). We show that VP16-B, while also being a powerful tra nsactivator of transcription dependent on Oct-1 binding sites in its t arget promoters, has virtually no activity on a defined VP16-H-respons ive, octamer-containing target promoter. While Oct-1 binds equally wel l to the VP16-B-responsive and -nonresponsive sites, VP16-B interacts with Oct-1 only when Oct-1 is bound to the BHV octamer site and not wh en it is bound to the HSV site. We show from the analysis of chimeric proteins that the ability of VP16-B to discriminate between the Oct-1 forms depends on features of its N-terminal region. We also show from an analysis of chimeric DNA motifs that sequences that lie 3' to the P OU domain-contacting region of the HSV octamer site play a role in mak ing it unresponsive to VP16-B. Finally, we show by high-resolution hyd roxyl radical footprint analysis that the conformation of Oct-1 is dif ferent on the two sites. These results augment our previous report on an allosteric effect of DNA signals on the conformation of bound prote ins and indicate that different conformations of the same DNA binding protein can be recognized selectively by related members of interactin g regulatory proteins. The possible implications of our observations f or selective gene regulation by Oct-1, a ubiquitous transcription fact or, and other multimember transcription families are discussed.