In Drosophila beta tubulins are encoded by a small gene family whose members are differentially expressed in a highly cell and tissue specific manner. Here we focus on the expression of the beta 3 tubulin isotype during mesoderm differentiation and beta 1 tubulin expression in the apodemes during embryonic development. The beta 3 tubulin isotype is first detectable at the extended germband stage shortly before the separation of somatic and visceral derivatives. Comparing the distribution of the

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... beta 3 mRNA and the beta 3 isotype shows that the transcription of the beta 3 tubulin gene is cell type specifically repressed during differentiation of individual mesodermal derivatives, from which the dorsal vessel remains transcriptionally active until shortly before hatching. In contrast the beta 3 tubulin protein is detectable in all mesodermal derivatives. The beta 3 tubulin is an excellent marker to study mesoderm differentiation on a regulatory and cellular level using both genetics and molecular biology. In the visceral mesoderm, the expression of the beta 3 tubulin gene is regulated by homeotic gene products, while other transactivators regulate expression in the dorsal vessel and the body wall musculature. In the somatic mesoderm, the beta 3 tubulin allows to visualize myotube formation and insertion into the epidermis. This contact to the epidermal attachment sites (apodemes) induces beta 1 tubulin expression, as can be seen in double staining experiments. We determined a 14bp cis-regulatory enhancer element guiding expression of the beta 1 tubulin gene in these attachment sites. Using the beta 1 and beta 3 tubulin isotypes as markers we started to isolate mutants which are disturbed in muscle formation.