Formation of transcription preinitiation complexes with an amanitin-resistant RNA polymerase II.

Accurate transcription by RNA polymerase II has been shown to require multiple factors which participate in a number of intermediate steps prior to initiation. The last detectable step before initiation is the formation of an activated or rapid start complex which is template-associated. In this study we used two phenotypically distinguishable forms of mammalian RNA polymerase II to examine the requirement for specific factors in the formation of the activated complex. RNA polymerase II was purified from a mutant cell line which was resistant to levels of alpha-amanitin that are toxic for normal cells. When added to a polymerase-dependent transcription system consisting of HeLa factors and adenovirus DNA, the mutant polymerase accurately transcribed the template in an amanitin-resistant fashion. This amanitin-resistant transcription was competitively inhibited when wild-type polymerase was also added to the system. Preincubation of amanitin-resistant polymerase with DNA and factors produced a polymerase-associated complex, defined by its resistance to exchange for the amanitin-sensitive polymerase. Complex formation required the presence of polymerase, DNA, and the HeLa factors during the preincubation but did not require the presence of nucleotides. Complexes were template-associated, as shown by their inability to exchange onto a second template. Thus, prior to initiation, RNA polymerase II forms a stable association with the DNA template in an activated complex.