INTRODUCTION Transcriptional regulation plays a major role in the expression of the genomic information during complex biological processes such as differentiation and development. The effect of binding of transcription factors to cis-acting DNA sequences

Transcriptional regulation plays a major role in the expression of the genomic information during complex biological processes such as differentiation and development. The effect of binding of transcription factors to cis-acting DNA sequences must somehow be transmitted to RNA polymerase to ensure the initiation and maintenance of active transcription. A class of proteins called co-activators serve this role by recruiting the general transcription machinery and by modifying the chromatin structure (Näär et al., 2001; Roeder, 1991). Although sequence-specific DNA-binding transcription factors provide most of the spatial and temporal specificity of expression, the expressional and functional properties of coactivators are also likely to participate in determining the kinetics and efficiency of transcription. Two classes of transcriptional co-activators have been described to date. The first class comprises proteins that possess or recruit enzymatic activities to modify chromatin proteins, e.g. by acetylation of histones. Resulting alteration in the chromatin structure causes a switch in the ‘state’ of chromatin between transcriptionally inactive and active. Coactivators of the second class act more directly to recruit the general transcription machinery to a promoter where a transcription factor is bound. Among the latter class are TATA element-binding protein (TBP)-associated factors (TAFs) that are subunits of TFIID, and others that serve as adaptors to mediate the contact between transcription factors and the basal transcriptional complex. Although the importance of the first type co-activators for gene expression has been demonstrated (Akimaru et al., 1997; Brownell et al., 1996; Chakravarti et al., 1996; Grant et al., 1997; Kamei et al., 1996; Ogryzko et al., 1996; Waltzer and Bienz, 1999), little is known about how the second class co-activators function in vivo, or how the two types interact to achieve elaborate regulation of gene expression. Multiprotein bridging factor 1 (MBF1) was first identified from the silkmoth as a co-factor necessary for transcriptional activation in vitro by a nuclear receptor FTZ-F1 (Li et al., 1994; Takemaru et al., 1997). The ability of MBF1 to bind both FTZF1 and TBP suggested a mechanistic model in which MBF1 recruits TBP to a promoter carrying the FTZ-F1-binding site by interconnecting FTZ-F1 and TBP. The MBF1 sequence is highly conserved across species from yeast to human (Takemaru et al., 1997). In the yeast Saccharomyces cerevisiae, MBF1 functions as a co-activator of a bZIP transcription factor GCN4, by bridging between GCN4 and TBP in response to amino acid starvation (Takemaru et al., 1998). To analyze the biological role of MBF1 in a multicellular organism, we characterized its ortholog in Drosophila. The Drosophila mbf1 gene partially rescued the phenotype of a S. cerevisiae mbf1– mutant, establishing that both the structure 719 Development 130, 719-728 © 2003 The Company of Biologists Ltd doi:10.1242/dev.00297