Characterisation of the promoter region of the zebrafish six7 gene.

The Drosophila homeobox gene sine oculis and its murine homologue Six3 have both been shown to have regulatory functions in eye and brain development. In zebrafish, three Six3-related genes with conserved expression during early eye and head formation have been identified. One of these, six7, is first expressed at the gastrula stage in the involuting axial mesoderm, and later in the overlying neuroectoderm from which the forebrain and optic primordium develop. To elucidate the mechanisms regulating six7 expression, we isolated a 2.7-kb fragment of the 5'-flanking region. Three sequentially deleted fragments of this upstream region were used to produce GFP reporter constructs for analysis of tissue-specific expression in zebrafish embryos. The results show that a 625-bp upstream fragment is sufficient to direct strong expression of the reporter during gastrulation and early neurulation. The proximal part of the promoter contains binding sites for various constitutive transcription factors and an additional upstream element that was shown to be critical in directing expression to the anterior region of the zebrafish brain.