Figure 5.

A working model explaining the requirement of CTD-phosphorylated RNAP IIO for high levels of transcription activation in unfractionated HeLa NEs. Transcription by CTD-hyperphosphorylated RNAP IIO and unphosphorylated RNAP IIA may be equally repressed by negative (co)factors that are present in unfractionated HeLa NEs but that are absent in systems reconstituted with purified components. Transcription activators may selectively stimulate RNAP IIO activity either directly and/or indirectly by reversing the effect of negative factors (see Discussion).