<> "The repository administrator has not yet configured an RDF license."^^ . <> . . "Drosophila myogenesis as a model for studying cis-regulatory networks : identifying novel players and dissecting the role of transcriptional repression"^^ . "Recent studies have identified in vivo binding profiles of key mesodermal regulators across the Drosophila melanogaster genome. Many of the occupied sites lie in the vicinity of loci encoding yet other transcription factors. The analyzed cis- regulatory modules drive expression in a variety of complex spatio-temporal patterns that cannot be explained by the binding of the core regulators alone. Thus there clearly are additional, unknown transcription factors in the regulatory network that governs the process of embryonic mesoderm specification and muscle differentiation. In order to identify novel myogenic regulators in a systematic way, and thereby enrich the underlying network, I initiated a molecular screen to uncover new players. Candidate putative transcription factors were prioritized based on their expression in mesoderm and on available ChIP-on-chip and expression profiling data. Their role in myogenesis was subsequently assayed using Drosophila deficiency lines whose phenotypes were analyzed with a muscle-specific marker. Altogether, 67 different deficiency and loss-of-function lines were used individually or in combination to delete 46 transcription factors with mesodermal expression. In 21 of the 46 cases, the mutant embryos displayed specific defects in the development of one or more muscle types. One pair of partially overlapping deficiencies placed in trans showed a failure in myoblast fusion, a process that gives rise to muscle syncytia from mononucleated myoblasts. The corresponding deleted candidate gene was MED24, a subunit of the Mediator complex, which is a general co-activator of transcription. Muscle-specific knockdown of MED24 or MED14, another subunit of the complex whose deletion by deficiency lines phenocopies that of MED24, leads to lethality. To establish whether MED24 and MED14 are indeed involved in muscle development, I generated smaller deletion lines using FRT-mediated recombination. While deletion of MED14 does not affect myogenesis, embryos deficient for MED24 display supernumerary mononucleated myoblasts. Both small deletion lines were then combined together to detect possible redundancy that could obscure the requirement of MED14 and MED24 in muscle development. Another candidate transcription factor within the myogenic network based on ChIP-on-chip experiments is the transcriptional repressor Tramtrack69 (Ttk69). Ttk69 is expressed in the primordium of visceral and, more transiently, somatic muscle. In ttk69 mutant embryos, homozygous for a loss-of-function allele, myoblast fusion is delayed, the myoblasts aggregate in clusters, and fail to migrate towards the ectodermal attachment sites. Two distinct myoblast populations, a founder cell and multiple fusion competent myoblasts, contribute to each muscle fibre. As revealed by immunohistochemistry and in situ hybridization, in ttk69 mutants there are significantly more founder cells formed while the number of fusion competent myoblasts is decreased. Consistently, ectopic expression of Ttk69 in the founder cells, but not fusion competent myoblasts, gives rise to severe myoblast fusion defects. These phenotypic analyses suggest a model where Ttk69 is required for specification of fusion competent myoblasts and in its absence, their conversion to a founder cell-like fate may occur. According to the proposed model, Ttk69 would repress founder cell genes within the fusion competent myoblasts. To determine whether this holds true on a global scale, I performed a high-resolution ChIP-on-chip experiment in 6-8 hour wild type embryos. Indeed, Ttk69 binding was significantly enriched in the vicinity of founder cell-specific genes as compared to fusion competent myoblast-specific genes. ChIP-on-chip data generated for Lame duck, a transcriptional activator essential for fusion competent myoblast determination, showed the opposite tendency. It therefore appears that proper specification of fusion competent myoblast identity requires both positive input from Lame duck and inhibition of founder cell-specific genes by Ttk69. These findings advance our limited knowledge about the role of transcriptional repression within the myogenic regulatory network. Finally, I re-evaluated the role of Snail, a well-established transcriptional repressor involved in early mesoderm specification and gastrulation. Multiple observations suggested that Snail may also play a positive role in regulating mesodermal genes. To investigate this possibility, I performed luciferase assays with previously characterized mesodermal enhancers and showed that Snail can elevate their activation levels. In one case, this ability of Snail was suppressed upon mutagenesis of putative Snail binding motifs, both in cell culture and in vivo. Moreover, expression of the enhancers and their associated genes is significantly reduced in snail mutant embryos. Snail thus seems to play a dual role in repressing non-mesodermal genes, but also in contributing to the activation of some early mesodermal genes. "^^ . "2010" . . . . . . . . "Lucia"^^ . "Ciglar"^^ . "Lucia Ciglar"^^ . . . . . . "Drosophila myogenesis as a model for studying cis-regulatory networks : identifying novel players and dissecting the role of transcriptional repression (PDF)"^^ . . . "PhD_Thesis_Ciglar.pdf"^^ . . . "Drosophila myogenesis as a model for studying cis-regulatory networks : identifying novel players and dissecting the role of transcriptional repression (Other)"^^ . . . . . . "indexcodes.txt"^^ . . . "Drosophila myogenesis as a model for studying cis-regulatory networks : identifying novel players and dissecting the role of transcriptional repression (Other)"^^ . . . . . . "lightbox.jpg"^^ . . . "Drosophila myogenesis as a model for studying cis-regulatory networks : identifying novel players and dissecting the role of transcriptional repression (Other)"^^ . . . . . . "preview.jpg"^^ . . . "Drosophila myogenesis as a model for studying cis-regulatory networks : identifying novel players and dissecting the role of transcriptional repression (Other)"^^ . . . . . . "medium.jpg"^^ . . . "Drosophila myogenesis as a model for studying cis-regulatory networks : identifying novel players and dissecting the role of transcriptional repression (Other)"^^ . . . . . . "small.jpg"^^ . . "HTML Summary of #10800 \n\nDrosophila myogenesis as a model for studying cis-regulatory networks : identifying novel players and dissecting the role of transcriptional repression\n\n" . "text/html" . . . "570 Biowissenschaften, Biologie"@de . "570 Life sciences"@en . .