Abstract
BACKGROUND: The chromatin remodelers of the SWI/SNF family are critical transcriptional regulators. Recognition of lysine acetylation through a bromodomain (BRD) component is key to SWI/SNF function; in most eukaryotes, this function is attributed to SNF2/Brg1. RESULTS: Using affinity purification coupled to mass spectrometry (AP-MS) we identified members of a SWI/SNF complex (SWI/SNF(Tt)) in Tetrahymena thermophila. SWI/SNF(Tt) is composed of 11 proteins, Snf5(Tt), Swi1(Tt), Swi3(Tt), Snf12(Tt), Brg1(Tt), two proteins with potential chromatin-interacting domains and four proteins without orthologs to SWI/SNF proteins in yeast or mammals. SWI/SNF(Tt) subunits localize exclusively to the transcriptionally active macronucleus during growth and development, consistent with a role in transcription. While Tetrahymena Brg1 does not contain a BRD, our AP-MS results identified a BRD-containing SWI/SNF(Tt) component, Ibd1 that associates with SWI/SNF(Tt) during growth but not development. AP-MS analysis of epitope-tagged Ibd1 revealed it to be a subunit of several additional protein complexes, including putative SWR(Tt), and SAGA(Tt) complexes as well as a putative H3K4-specific histone methyl transferase complex. Recombinant Ibd1 recognizes acetyl-lysine marks on histones correlated with active transcription. Consistent with our AP-MS and histone array data suggesting a role in regulation of gene expression, ChIP-Seq analysis of Ibd1 indicated that it primarily binds near promoters and within gene bodies of highly expressed genes during growth. CONCLUSIONS: Our results suggest that through recognizing specific histones marks, Ibd1 targets active chromatin regions of highly expressed genes in Tetrahymena where it subsequently might coordinate the recruitment of several chromatin-remodeling complexes to regulate the transcriptional landscape of vegetatively growing Tetrahymena cells.