Swi/Snf is directly required for repression of transcription.
Joseph A. Martens, Fred Winston
Department of Genetics,
Harvard Medical School, 200 Longwood Ave., Boston, MA 02115, USA
The
Swi/Snf nucleosome-remodeling complex has been extensively characterized
as a transcriptional coactivator. However, more recent genetic and
whole-genome expression studies have suggested that Swi/Snf and related
nucleosome-remodeling complexes can also act as repressors of
transcription. To further investigate the role of Swi/Snf in
transcriptional repression, we have studied the SER3 gene from
S.cerevisiae, which encodes a serine biosynthetic enzyme.
SER3 is strongly repressed by Swi/Snf as SER3 mRNA levels
are increased approximately 50-fold in a snf2delta mutant. To
test if Swi/Snf represses SER3 directly, we performed chromatin
immunoprecipitation experiments, which demonstrated the presence of
Swi/Snf at the SER3 promoter. By micrococcal nuclease assay, we
show that the chromatin structure over the SER3 promoter is
altered in a snf2delta mutant, suggesting that Swi/Snf maintains
a repressive chromatin structure over the SER3 promoter. In
striking contrast to activation by Swi/Snf, which requires most Swi/Snf
subunits, repression by Swi/Snf at SER3 is primarily dependent
upon only one Swi/Snf component, Swi2/Snf2. Taken together, these
results strongly suggest that Swi/Snf can directly repress transcription
in vivo and that the mechanism of repression has distinct
differences from the mechanism of Swi/Snf activation.
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