MUSCLE DEVELOPMENT AND ATTACHMENT TO THE EPIDERMIS IS ACCOMPANIED BY EXPRESSION OF BETA-3 AND BETA-1 TUBULIN ISOTYPES, RESPECTIVELY

In Drosophila beta tubulins are encoded by a small gene family whose m embers are differentially expressed in a highly cell and tissue specif ic manner. Here we focus on the expression of the beta 3 tubulin isoty pe during mesoderm differentiation and beta 1 tubulin expression in th e apodemes during embryonic development. The beta 3 tubulin isotype is first detectable at the extended germband stage shortly before the se paration of somatic and visceral derivatives. Comparing the distributi on of the beta 3 mRNA and the beta 3 isotype shows that the transcript ion of the beta 3 tubulin gene is cell type specifically repressed dur ing differentiation of individual mesodermal derivatives, from which t he 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 usin g both genetics and molecular biology. In the visceral mesoderm, the e xpression of the beta 3 tobulin gene is regulated by homeotic gene pro ducts, while other transactivators regulate expression in the dorsal v essel and the body wall musculature. In the somatic mesoderm, the beta 3 tubulin allows to visualize myotube formation and insertion into th e 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 guid ing expression of the beta 1 tubulin gene in these attachment sites. U sing the beta 1 and beta 3 tubulin isotypes as markers we started to i solate mutants which are disturbed in muscle formation.