Upstream nucleosomes and Rgr1p are required for nucleosomal repression of transcription

The mechanisms of transcription repression and derepression in vivo are not fully understood. We have obtained evidence that begins to clarify the min imum requirements for counteracting nucleosomal repression in vivo. Locatio n of the TATA element near the nucleosome dyad does not block RNA polymeras e II transcription in vivo if there is a nucleosome-free region located imm ediately upstream. However, location of the TATA element similarly within t he nucleosome does block transcription if the region upstream of it is nucl eosome bound. Histone H4 depletion derepresses transcription in the latter case, supporting the idea that the nucleosomes are responsible for the repr ession. These results raise the intriguing possibility that the minimum req uirement for derepression of transcription in vivo is a nucleosome-free reg ion upstream of the core promoter. Importantly, we find that a C-terminal d eletion in RGR1, a component of the mediator/holoenzyme complex and a globa l repressor, can also derepress transcription.