RESOLUTION OF RNA-POLYMERASE-I INTO DIMERS AND MONOMERS AND THEIR FUNCTION IN TRANSCRIPTION

We have further analyzed the requirements of yeast RNA polymerase I (p ol I) to initiate transcription at the ribosomal gene promoter. Resolu tion of yeast whole cell extracts through several chromatographic step s yielded three protein fractions required for accurate initiation. On e fraction is composed of TBP associated within a 240 kDa protein comp lex. The fraction contributing the RNA polymerase I (pol I) activity c onsists of dimeric and monomeric pol I under conditions optimal for in vitro transcription. The capability to utilize the ribosomal gene pro moter correlates with monomeric pol I complexes which are possibly ass ociated with further transcription factors. These initiation competent pol I complexes appeared to be resistant to high salt concentrations. Pol I dimers which represent the majority of the isolated pol I, can be reversibly dissociated into monomers and are only active in non-spe cific RNA synthesis, if single stranded DNA serves as a template. We s uggest a model in which dimeric inactive pol I is converted into an ac tive monomeric form that might be associated with other transcription factors to maintain a stable initiation competent complex.