Institut fuer Allgmeine Genetik (240), Universitaet Hohenheim, D- 70593 Stuttgart, F.R.G.
Neurogenesis in Drosophila is a two step process: at first, proneural gene activity defines neural competence groups from which in the second step, presumptive neuroblasts are selected by a lateral inhibition process. In this process, cell-cell-communication occures through the Notch- pathway, at the end of which basic helix loop helix (bHLH)-genes of the E(spl)-complex are thought to repress genes required for the neuronal fate.
The E(spl)-complex contains at least 14 transcription units spanning about 60kb. Within the complex there are 7 structurally related genes. Each of them contains 3 functionally important domains, a bHLH domain, an "orange"-domain and the tetrapeptide WRPW at the C-terminus. It is thought that they form dimers and serve as transcriptional regulators via the bHLH and presumably also the "orange"-domain. The WRPW-terminus is an interactive domain bound by Groucho (m9/10), another member of the complex.
Genetic approaches have shown that none of these E(spl)-bHLH-gene-products are required individually for fly-survival, indicating a high degree of functional redundancy. Their concomitant loss causes an extreme neuronal hyperplasia. The high degree of conservation of the whole complex between Drosophila melanogaster and Drosophila hydei suggests important functions of each of its transcription units.
In order to obtain further information on the function of the single transcription units, we intended to produce smaller deletions in the E(spl)-complex. We used X-rays and imprecise excisions of a P-element (jump outs), located near the 5prime end of the m7 E(spl)bHLH gene to obtain such deletions. We analysed 200 jump out lines, two of which carried a deletion of m7. These mutants are homozygous viable and do not show any visible phenotypes. It seems that the function of m7 can be fullfilled otherwise, presumably by genes of the E(spl)-complex. Using X-ray, we irradiated 9000 male flies and analysed their offspring. We obtained 8 lines with deletions in the E(spl)-region. All of them are homozygous lethal. Currently we are investigating the molecular limits of these mutations in more detail.