Tinman Regulates the Transcription of the β3 tubulin Gene (βTub60D) in the Dorsal Vessel of Drosophila

Abstract

During Drosophila embryogenesis, the β3 tubulin gene is expressed in the visceral and somatic mesoderm as well as in the dorsal vessel. Transcription of the gene is limited to four pairs of cardioblasts per segment. Here we show that its expression in the dorsal vessel (dv) is mediated by a 333-bp enhancer located upstream of the gene. The homeodomain protein Tinman is also expressed in these cardioblasts, implying that Tinman might be a key regulator of the β3 tubulin gene. Gel retardation and footprint assays indeed revealed two Tinman binding sites within the dv-specific enhancer. We analyzed the relevance of the Tinman binding sites in a transgenic fly assay and observed distinct functions for both sites. The BSTin-1460 site is absolutely required for expression in cardioblasts, while BSTin-1425 is needed for high-level expression. Thus, these two Tinman binding sites act in concert to drive β3 tubulin gene expression during heart development. Tinman initially functions in the specification of visceral mesoderm and heart progenitors, but remains expressed in cardioblasts until dorsal closure. Overall, our data demonstrate a late function for Tinman in the regulation of β3 tubulin gene expression in the forming heart of Drosophila.