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Abstract

The embryonal development of higher organisms is conducted by a network of many different structural and regulatory gene functions whose complexity is yet poorly understood. Some genetic elements identified as being involved in specifying segment identity of the fruit fly Drosophila melanogaster are the homeotic genes (Ouweneel 1976). Several homeotic genes of Drosophila belonging to either the engrailed gene complex (EN-C; Garcia-Bellido and Santamaria 1972) or the antennapedia and bithorax complex (ANT-C/BX-C; Regulski et al. 1985) contain a 180 bp conserved region, the “homeo box”, coding for a protein domain of the helix-turn-helix type (McGinnis et al. 1984a, b; Scott and Weiner 1984). This type of DNA-binding domain was first detected in procaryotes. Interestingly, the two yeast mating-type proteins MAT a-1 and MAT α2 (Shepherd et al. 1984; Laughon and Scott 1984) show significant similarities to the consensus homeodomain sequence. MAT α2 is a transcriptional repressor (Wilson and Hershkowitz 1984; Hartig et al. 1986). It was therefore hypothesized that the Drosophila homeodomain proteins might also have regulatory functions, e.g., by interacting directly or indirectly in trans with their own promoter regions or those of other genes (Desplan et al. 1985).