FIG. 1.

Sex-determining hierarchy of D. melanogaster and DM motif. (A) X-to-autosome ratio regulates a sex-specific RNA-splicing cascade (50). Not shown are Intersex (IX), a putative transcriptional coactivator that acts with DSX^F to promote female development, and a divergent fruitless branch downstream of tra factors. (B) Domain organization of DSX isoforms N-terminal DM and C-terminal dimerization domains. DSX^M and DSX^F differ at the extreme C terminus (cross-hatched and gray regions). (C) Ribbon model of DM domain with intertwined CCHC and HCCC Zn²⁺-binding sites (DSX residues 41 to 81) (68). (D) Metazoan DM sequences. Cysteines and histidines that coordinate Zn²⁺ are aligned (red and green boxes). Stable α-helical elements are highlighted in magenta; the nascent helical tail is shown in gray. The arrow highlights residue K60, conserved on the surface of the Zn module and proposed to contact DNA.