SCRATCH, A PAN-NEURAL GENE ENCODING A ZINC-FINGER PROTEIN RELATED TO SNAIL, PROMOTES NEURONAL DEVELOPMENT

The Drosophila scratch (scrt) gene is expressed in most or all neurona l precursor cells and encodes a predicted zinc finger transcription fa ctor closely related to the product of the mesoderm determination gene snail (sna). Adult flies homozygous for scrt null alleles have a redu ced number of photoreceptors in the eye, and embryos lacking the funct ion of both scrt and the pan-neural gene deadpan (dpn), which encodes a basic helix-loop-helix (bHLH) protein, exhibit a significant loss of neurons. Conversely, ectopic expression of a scrt transgene during em bryonic and adult development leads to the production of supernumerary neurons. Consistent with scrt functioning as a transcription factor, various genes are more broadly expressed than normal in scrt null muta nts. Reciprocally, these same genes are expressed at reduced levels in response to ectopic scrt expression. We propose that scrt promotes ne uronal cell fates by suppressing expression of genes promoting non-neu ronal cell fates. We discuss the similarities between the roles of the ancestrally related scrt, sna, and escargot (esc) genes in regulating cell fate choices.