Identification of engrailed promoter elements essential for interactions with a stripe enhancer in Drosophila embryos.

The structures and functions of promoter sequences of most genes have been analysed using in vitro transcription and/or cultured cell systems, neither possessing tissue-specific enhancers. Promoter-enhancer interactions in vivo, in particular, during ontogeny, are still poorly understood. We have established a new method for the assessment of promoter activity in cells that participate in fly body formation, using the UAS/GAL4 system. A functional analysis was then conducted on the promoter sequence of the engrailed gene in Drosophila embryos. A 38-bp-long sequence, terminating with an initiator or RNA start site and a downstream promoter element, was found to be capable of receiving activation signals from the engrailed stripe enhancer. Transcriptional efficiency was improved significantly by the presence of upstream promoting elements, most functionally replaceable with synthetic GAGA factor binding sites. We identified the in vivo minimum promoter of engrailed and demonstrated that the GAGA factor binding sites serve primarily as quantitative elements which augment transcriptional efficiency. Evidence was also obtained that indicated that not only enhancer but also promoter sequences were involved in the determination of the tissue-specificity of gene expression.