Genetic Characterization of the Homeodomain-Independent Activity of the Drosophila Fushi Tarazu Gene Product

The gene product of fushi tarazu (FTZ) has a homeodomain (HD)-independent activity. Ectopic expression of a FTZ protein that lacks half the HD in embryos results in the anti-ftz phenotype. We have characterized this FTZ HD-independent activity further. Ectopic expression of the HD-independent FTZ activity, in the absence of FTZ activity expressed from the endogenous ftz gene, was sufficient to result in the anti-ftz phenotype. Since the anti-ftz phenotype is a first instar larvae composed nearly entirely of FTZ-dependent cuticular structures derived from the even-numbered parasegments, this result suggests that expression of the HD-independent FTZ activity is sufficient to establish FTZ-dependent cuticle. Activation of FTZ-dependent Engrailed (EN) expression and activation of the ftz enhancer were HD-independent. The ftz enhancer element, AE-1, was activated by the HD-independent FTZ activity; however, the ftz enhancer element, AE-BS2CCC, which is the same as AE-1 except for the inactivation of two FTZ HD DNA-binding sites, was not. Activation of the ftz enhancer by ectopic expression of FTZ activity was effective only during gastrulation and germ band extension. In the discussion, we propose an explanation for these results.     

Gene activation and DNA binding by Drosophila Ubx and abd-A proteins

The Ubx and abd-A gene products are required for proper development of thoracic and abdominal structures in Drosophila. We expressed LexA-Ubx and LexA-abdA fusion proteins in yeast. These proteins activated expression of target genes that carried either upstream LexA operators or upstream Ubx binding sites. Both proteins contain homeodomains. Experiments with mutant fusion proteins show that the homeodomain is not required for the proteins to form dimers or enter the nucleus, and that, when DNA binding is provided by the LexA moiety, the homeodomain is not required for gene activation. Our results suggest that the homeodomain is necessary for these proteins to bind Ubx sites, but that the homeodomain does not contact DNA exactly like bacterial helix-turn-helix proteins. Finally, our data suggest that gene activation by these proteins is a simple consequence of their binding to DNA, while negative gene regulation requires that these proteins act together with other Drosophila gene products.