THE BIPARTITE DROSOPHILA-MELANOGASTER TWIST PROMOTER IS REORGANIZED IN DROSOPHILA-VIRILIS

The pivotal role of twist in mesoderm determination in the Drosophila embryo depends upon two processes-the transcriptional activation of tw ist in the ventrally located mesodermal anlage and the regulation of d ownstream gene expression by the twist transcription factor. To elucid ate the molecular mechanisms involved in these processes, we have comp ared both the coding and regulatory regions of the twist genes from Dr osophila melanogaster and Drosophila virilis. Within the coding region , the basic-helix-loop-helix DNA binding and dimerization motif is hig hly conserved, consistent with the functional importance of this domai n. A comparison of the transcriptional regulatory regions reveals a hi gh degree of conservation in the more distal of the two ventral activa tor regions that have been mapped in the twist 5' flanking region. On the other hand, the more proximal ventral activator region is absent a t the corresponding position in the D. virilis twist gene. Instead, th ere is a region in the second intron of the D. virilis gene that resem bles the proximal element of the D. melanogaster gene, in that it cons ists of little more than a series of whole and half binding sites for the dorsal morphogen. In transformation experiments, the intronic D. v irilis element directs an expression pattern that is indistinguishable from that directed by the D. melanogaster proximal VAR. Thus, the twi genes from these two species appear to have evolved enhancer elements with very similar structural and functional properties. These finding s suggest that apparently redundant spatially regulated enhancer eleme nts may each play essential roles in fine tuning the level and/or patt ern of gene expression.