Mapping functional specificity in the Dfd and Ubx homeo domains.

To define homeo domain subregions that are important for embryonic targeting specificity of homeotic proteins, we generated a series of Deformed/Ultrabithorax chimeric genes in which parts of the Deformed homeo box region were substituted with Ultrabithorax sequences. Chimeric coding regions were attached to heat shock promoters and introduced into the Drosophila genome by P-element transformation. After heat-induced ectopic expression in embryos, we examined the cuticular phenotypes induced by the resulting chimeric proteins. We also tested the ability of the chimeric proteins to regulate transcription units that are normal targets of Deformed and Ultrabithorax. Our results indicate that specific amino acid residues at the amino end of the Ultrabithorax homeo domain are required to specifically regulate Antennapedia transcription; and in the context of a Deformed protein, these amino-end residues are sufficient to switch from Deformed- to Ultrabithorax-like targeting specificity. Although residues in the amino end of the homeo domain are also important in determining a Deformed-like targeting specificity, other regions of the Deformed homeo domain are also required for full activity.