TRANSCRIPTIONAL ACTIVATION DOMAINS OF THE SINGLE-MINDED BHLH PROTEIN ARE REQUIRED FOR CNS MIDLINE CELL-DEVELOPMENT

The single-minded gene functions as a master developmental regulator w ithin the midline cell lineage of the embryonic central nervous system of Drosophila melanogaster. Genetic experiments suggest that Single-m inded can function as a transcriptional activator. Regions of the Sing le-minded protein were fused to the DNA binding domain of the mammalia n transcription factor Spl and shown to activate transcription from a reporter gene linked to Spl binding sites. Three independent activatio n domains were identified in the carboxy terminal region of Single-min ded that include areas rich in serine, threonine, glutamine and prolin e residues. Germ line transformation experiments indicate that the car boxy terminal activation domains, the PAS dimerization domain, and the putative DNA binding basic domain of Single-minded are required for e xpression of CNS midline genes in vivo. These results define in vivo a functional activation domain within Single-minded and suggest a model in which Single-minded activates transcription through a direct inter action with promoter elements of CNS midline genes.