Transcription activation by RNA polymerase II is a complicated process driven by combined, precisely coordinated action of a wide array of coactivator complexes, which carry out chromatin-directed activities and nucleate the assembly of the preinitiation complex on the promoter. Using various techniques, we have shown the existence of a stable coactivator supercomplex consisting of the chromatin-remodeling factor Brahma (SWI/SNF) and the transcription initiation factor TFIID, named BTFly (Brahma and TFIID in one assembly). The coupling of Brahma and TFIID is mediated by the SAYP factor, whose evolutionarily conserved activation domain SAY can directly bind to both BAP170 subunit of Brahma and TAF5 subunit of TFIID. The integrity of BTFly is crucial for its ability to activate transcription. BTFly is distributed genome-wide and appears to be a means of effective transcription activation.coactivators ͉ protein complex A ctivation of transcription by eukaryotic RNA polymerase II (Pol II) requires different groups of coactivators (for reviews, see refs. 1 and 2). The primary function of coactivators is to remodel and modify the chromatin template. Thus, chromatin remodelers of the Brahma (SWI/SNF-related) family play a genome-wide role in activation of Pol II-transcribed genes (3, 4). One more function of coactivators is to further recruit general transcription factors (GTFs) to form the Pol II preinitiation complex. The TFIID coactivator performs this function for most of Pol II-dependent genes (5, 6).Different coactivators recruited to the promoter assist each other and interact in a highly organized gene-specific manner (for a review, see ref. 7). However, this important regulatory step is still poorly understood. The best studied model is that of successive one-by-one recruitment of coactivators, which, in particular, is confirmed by the fact that the recruitment of chromatin-remodeling complexes is usually a prerequisite for the efficient recruitment of GTFs to the promoter (8, 9). The opposite model proposes one-time recruitment of preexisting supercomplex of several coactivators (10-12), although the composition of such supercomplexes described to date appears to be either ambiguous or incomplete.We have described the coactivator SAYP in Drosophila (13). SAYP is present at numerous sites on polytene chromosomes and colocalizes with Pol II in transcriptionally active euchromatin. SAYP homologs in various metazoans have an evolutionarily conserved core containing the SAY domain, which is involved in transcription activation, and 2 PHD fingers (13). Recently, SAYP was found to be associated with the chromatinremodeling Brahma complex of the PBAP subfamily (14). Here, we show that SAYP interacts both with Brahma and with TFIID, assembling them into a stable supercomplex named BTFly (Brahma and TFIID in one assembly). The presence of all BTFly components is crucial for its function in transcription activation. An important fact is that highly purified BTFly contains the full set of TFIID and Brahma subunits and, t...

SAYP is a dual-function transcription coactivator of RNA polymerase II. It is a metazoan-specific factor with regulated expression that is apparently involved in signaling pathways controlling normal development. In Drosophila, SAYP is maternally loaded into the embryo, participates in cell cycle synchronization in early syncytial embryos, and is indispensible for early embryogenesis. SAYP is abundant in many embryonic tissues and imaginal discs in larvae and is crucial for oogenesis in adults. PHF10 is a mammalian homologue of SAYP whose expression is confined to certain tissues in adults. The molecular mechanism of the SAYP function is related to the conserved domain SAY, which assembles a nuclear supercomplex BTFly consisting of Brahma and TFIID coactivators. We suggest that nuclear supercomplexes may be important means of gene-specific regulation of transcription during development.

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Jak/STAT is an important signaling pathway mediating multiple events in development. We describe participation of metazoan co-activator SAYP/PHF10 in this pathway downstream of STAT. The latter, via its activation domain, interacts with the conserved core of SAYP. STAT is associated with the SAYP-containing co-activator complex BTFly and recruits BTFly onto genes. SAYP is necessary for stimulating STAT-driven transcription of numerous genes. Mutation of SAYP leads to maldevelopments similar to those observed in STAT mutants. Thus, SAYP is a novel co-activator mediating the action of STAT.

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