Figure 2.

(A) Domain organization of DSX^M, DSX^F, and MAB-3. Amino-terminal black rectangles indicate DM domains; the carboxy-terminal DSX dimerization domain and sex-specific extension (cross-hatched or gray; not present in MAB-3) are also indicated. (B,top) Three transcription factors (AEF1, DSX, and bZIP1) bind fbe enhancer (An and Wensink 1995a,b) at overlapping target sites (aef1 is shown in black, dsxA in red, and bzip1 in blue); (bottom) consensus sequences. Boxed sites are based on DNase protection; DSX footprint spans 21 bp, whereas DmC/EBP footprint spans 19 bp. DNase footprints overestimate DNA target sites due to steric occlusion between enzyme and protein–DNA complex. (C) Model of sex- and tissue-specific regulation of yolk–protein expression by fbe (An and Wensink 1995a,b). DSX binds as dimer; only monomer is shown (ribbon model). In model dsxA and bzip1 are occupied simultaneously to activate promoter in female. Binding of either DSX^M or DSX^F displaces AEF1 from its target site aef1. In female, fat body expression of DSX^F is higher than that of AEF1, and therefore in the presence of bZIP1, yolk proteins are expressed. Expression is repressed in ovary with higher levels of AEF1, which displace DSX^F. Male-specific repression of yolk proteins occurs as binding of DSX^M, which is 122 residues longer than DSX^F (Burtis and Baker 1989), occludes bzip1 or inactivates bound bZIP1, and therefore is repressed in male (C,bottom; Coschigano and Wensink 1993). It is not known which Drosophila bZIP family member is active in fat body. (D) Overlapping DSX DM and bZIP phosphate contacts are inferred based on methylphosphonate interference studies (red circles); sites of bZIP contact are as predicted by GCN4 cocrystal structure (blue circles; Keller et al. 1995). Filled or half-filled red circles indicate sites of strong or weak DM interference, respectively.