INTERPLAY OF YEAST GLOBAL TRANSCRIPTIONAL REGULATORS SSN6P-TUP1P AND SWI-SNF AND THEIR EFFECT ON CHROMATIN STRUCTURE

Transcriptional regulation in yeast involves a number of general trans -acting factors affecting chromatin structure. The Swi-Snf complex is required for expression of a large number of genes and has the ability to remodel chromatin in vitro, The Ssn6p-Tup1p repressor complex may be involved in chromatin organization through the interaction with pat hway-specific DNA-binding proteins. To study the interplay of these fa ctors and their effect on chromatin we have analyzed SUC2 chromatin st ructure in wild-type cells and in strains bearing combinations of ssn6 /tup1 and swi1 mutations. We have mapped nucleosome positioning of the repressed gene in wild-type cells using primer extension methodology, allowing base pair resolution, and have analyzed details of chromatin remodeling in the derepressed state. In ssn6 or tup1 mutants under re pressing conditions the observed changes in SUC2 chromatin structure m ay be suppressed by the swi1 mutation, suggesting that Ssn6p-Tup1p is not required for the establishment of nucleosome positioning at the SU C2 promoter, Our data indicate the involvement of chromatin remodeling factors distinct from the Swi-Snf complex in SUC2 transcriptional reg ulation and suggest that Swi-Snf may antagonize Ssn6p-Tup1p by control ling remodeling activity. We also show that a relatively high level of SUC2 transcription can coexist with positioned nucleosomes.