Independent regulatory elements in the upstream region of the Drosophila beta 3 tubulin gene (beta Tub60D) guide expression in the dorsal vessel and the somatic muscles.

The beta 3 tubulin gene (beta Tub60D) is a structural gene expressed during mesoderm development from the extended germ band stage onward. Expression within the individual mesodermal derivatives is guided by different control elements. The upstream regions allow expression in the dorsal vessel and the somatic mesoderm while enhancers localized in the first intron guide expression in the visceral mesoderm. Deletion analysis carried out in transgenic flies revealed independent regulatory elements for the dorsal vessel and the somatic mesoderm. For expression in the somatic mesoderm, a 279-bp region is absolutely essential. This region contains a binding site for the Drosophila myocyte-specific enhancer binding factor 2 (D-MEF2), a MADS-box transcription factor known to be essential for mesoderm development. Deletion or mutation of this D-MEF2 binding site strongly reduces transcription. This pattern is consistent with the strongly reduced expression of beta 3 tubulin in D-mef2 mutant embryos. This analysis furthermore reveals that the D-MEF2 binding site acts in concert with nearby cis regulatory elements. These data show that the upstream control region of the beta 3 tubulin gene is an early target of the D-MEF2 transcriptional activator.