The shadow enhancer of short gastrulation also directs its expression in the ventral midline of the Drosophila embryo

Abstract

The sog, a fly homologue of the vertebrate chordin, is expressed in the presumptive neurogenic ectoderm and also the ventral midline during Drosophila embryogenesis. Previously, two enhancers, called the primary and shadow enhancer, respectively, were found to direct sog expression in the neurogenic ectoderm. However, efforts to identify an enhancer that directs sog expression in the ventral midline have failed. Here, we present evidence that the sog shadow enhancer has dual activities to direct sog expression in the neurogenic ectoderm as well as the ventral midline. Systematic truncation analyses employing transgenic embryos revealed that about a 680-bp region within the shadow enhancer is necessary and sufficient for midline enhancer activity. Intriguingly, midline enhancer activity does not appear to depend on binding sites for Sim, because distal and proximal elements critical for sog midline expression within the 680-bp region do not have any canonical Sim binding sites.