Mutational Analysis of the Drosophila homothorax Gene

The homothorax (hth) gene is involved in multiple aspects of embryonic and adult fly development. It encodes a homeodomain-containing protein of the MEIS family and was shown to regulate the subcellular localization of the homeotic protein cofactor Extradenticle (EXD). The HTH protein contains a TALE class homeodomain and a conserved MH domain, which is required for its interaction with EXD. In this work, we describe the structure of the hth locus, characterize at the molecular level a collection of mutant alleles of hth, and discuss the correlation between the identified structural defects and their consequent phenotypes. The hth locus spans more than 100 kb and contains 14 exons. Several of the exon-intron boundaries within the homeodomain and the MH domain-coding regions are conserved between Drosophila and Caenorhabditis elegans. The analysis of hth mutations demonstrates that the homeodomain of HTH is not required for nuclear localization of EXD and that the MH domain-containing first 240 residues are sufficient for nuclear localization of both EXD and HTH. Mutations that alter or delete the homeodomain cause only partial homeotic transformations in the PNS, whereas mutations affecting the MH domain cause distinct and more severe PNS phenotypes. These observations may suggest that driving nuclear localization of EXD is the main role of HTH in patterning the embryonic PNS. They may also suggest that homeodomain-defective HTH protein retains some of its transcription-regulating functions by binding DNA via its interaction with EXD.