3 FUNCTIONAL CLASSES OF TRANSCRIPTIONAL ACTIVATION DOMAINS

We have studied the abilities of different transactivation domains to stimulate the initiation and elongation (postinitiation) steps of RNA polymerase II transcription in vivo Nuclear run-on and RNase protectio n analyses revealed three classes of activation domains: Spl and CTF s timulated initiation (type I); human immunodeficiency virus type I Tat fused to a DNA binding domain stimulated predominantly elongation (ty pe IIA); and VP16, p53, and E2F1 stimulated both initiation and elonga tion (type IIB). A quadruple point mutation of VP16 converted it from a type IIB to a type I activator. Type I and type IIA activators syner gized with one another but not with type IIB activators. This observat ion implies that synergy can result from the concerted action of facto rs stimulating two different steps in transcription: initiation and el ongation. The functional differences between activators may be explain ed by the different contacts they make with general transcription fact ors. In support of this idea, we found a correlation between the abili ties of activators, including Tat, to stimulate elongation and their a bilities to bind TFIIH.