Chromosomal localization links the SIN3-RPD3 complex to the regulation of chromatin condensation, histone acetylation and gene expression.

Acetylation of core histone N-terminal tails influences chromatin condensation and transcription. To examine how the SIN3-RPD3 deacetylase complex contributes to these events in vivo, we examined binding of SIN3 and RPD3 to Drosophila salivary gland polytene chromosomes. The binding patterns of SIN3 and RPD3 were highly coincident, suggesting that the SIN3-RPD3 complex is the most abundant chromatin-bound RPD3 complex in salivary gland cells. SIN3- RPD3 binding was restricted to less condensed, hypoacetylated euchromatic interbands and was absent from moderately condensed, hyperacetylated euchromatic bands and highly condensed, differentially acetylated centric heterochromatin. Consistent with its demonstrated role in transcriptional repression, SIN3-RPD3 did not co-localize with RNA polymer ase II. Chromatin binding of the complex, mediated by SMRTER, decreased upon ecdysone-induced transcriptional activation but was restored when transcription was reduced. These results implicate SIN3-RPD3 in maintaining histone acetylation levels or patterns within less condensed chromatin domains and suggest that SIN3-RPD3 activity is required, in the absence of an activation signal, to repress transcription of particular genes within transcriptionally active chromatin domains.