Molecular genetic analysis of bithorax complex functions in Drosophila melanogaster

In Drosophlla melanogaster, the homeotic genes clustered in the bithorax complex (BX-C) are responsible for specifying individual metameric identities of the posterior thorax and abdomen. The BX-C genes, Ultrabithorax (Ubx), abdominal-A (abd-A) and Abdominal-B (Abd-B), encode a conserved family of homeodomain-containing transcription factors that differentially regulate the expression of other genes required for specific segmental features. Lethal Ubx, abd-A and Abd-B alleles disrupt one of the three protein-coding transcription units whereas nonlethal alleles affect cis-regulatory regions. By molecularly defining the transcriptional start site of abd-A, I have confirmed that all lethal abd-A alleles map within the abd-A transcription unit. These results, consistent with the findings for Ubx and Abd-B, support the idea that nonlethal abd-A Alleles affect transcriptional control regions. Abd-B encodes two structurally related proteins that share the same homeodomain but have distinct developmental roles. Previous studies have Indicated that the homeodomain Is the determinant of DNA binding specificity; therefore, ABD-B m and r proteins might be expected to have the same DNA binding specificity but different regulatory effects. I have used an in vivo functional assay to assess the contribution of other protein domains outside the ABD-B homeodomain. I have shown that ABD-B m and r proteins have similar intrinsic activities when ubiquitously expressed throughout embryos, suggesting that other localized factors contribute to ABD-B m and r functional differences during development.