THE DORSAL-RELATED IMMUNITY FACTOR (DIF) CAN DEFINE THE DORSAL-VENTRAL AXIS OF POLARITY IN THE DROSOPHILA EMBRYO

In Drosophila embryos, dorsal-ventral polarity is defined by a signal transduction pathway that regulates nuclear import of the Dorsal prote in. Dorsal protein's ability to act as a transcriptional activator of some zygotic genes and a repressor of others defines structure along t he dorsal-ventral axis, Dorsal is a member of a group of proteins, the Rel-homologous proteins, whose activity is regulated at the level of nuclear localization, Dif, a more recently identified Drosophila Rel-h omologue, has been proposed to act as a mediator of the immune respons e in Drosophila, In an effort to understand the function and regulatio n of Rel-homologous proteins in Drosophila, we have expressed Dif prot ein in Drosophila embryos derived from dorsal mutant mothers, We found that the Dif protein was capable of restoring embryonic dorsal-ventra l pattern elements and was able to define polarity correctly with resp ect to the orientation of the egg shell. This, together with the obser vation that the ability of Dif to restore a dorsal-ventral axis depend ed on the signal transduction pathway that normally regulates Dorsal, suggests that Dif protein formed a nuclear concentration gradient simi lar to that seen for Dorsal. By studying the expression of Dorsal targ et genes we found that Dif could activate the zygotic genes that Dorsa l activates and repress the genes repressed by Dorsal. Differences in the expression of these target genes, as well as the results from inte raction studies carried out in yeast, suggest that Dif is not capable of synergizing with the basic helix-loop-helix transcription factors w ith which Dorsal normally interacts, and thereby lacks an important co mponent of Dorsal-mediated pattern formation.