Molecular genetic analysis of Drosophila eyes absent mutants reveals an eye enhancer element

The eyes absent (eya) gene is critical for normal eye development in Drosop hila and is highly conserved to vertebrates. To define regions of the gene critical for eye function, we have defined the mutations in the four viable eya alleles. Two of these mutations are eye specific and undergo transvect ion with other mutations in the gene. These were found to be deletion mutat ions that remove regulatory sequence critical for eye cell expression of th e gene. Two other viable alleles cause a reduced eye phenotype and affect t he function of the gene in additional tissues, such as the ocelli. These mu tations were found to be insertion mutations of different transposable elem ents within the 5' UTR of the transcript. Detailed analysis of one of these revealed that the transposable element has become subject to regulation by eye enhancer sequences of the eya gene, disrupting normal expression of EY A in the eye. More extruded analysis of the deletion region in the eye-spec ific alleles indicated that the deleted region defines an enhancer that act ivates gene expression in eye progenitor cells. This enhancer is responsive to ectopic expression of the eyeless gene. This analysis has defined a cri tical regulatory region required for proper eye expression of the eya gene.