Multiple enhancers ensure precision of gap gene-expression patterns in the Drosophila embryo

Segmentation of the Drosophila embryo begins with the establishment of spatially restricted gap gene-expression patterns in response to broad gradients of maternal transcription factors, such as Bicoid. Numerous studies have documented the fidelity of these expression patterns, even when embryos are subjected to genetic or environmental stress, but the underlying mechanisms for this transcriptional precision are uncertain. Here we present evidence that every gap gene contains multiple enhancers with overlapping activities to produce authentic patterns of gene expression. For example, a recently identified hunchback (hb) enhancer (located 5-kb upstream of the classic enhancer) ensures repression at the anterior pole. The combination of intronic and 5′ knirps (kni) enhancers produces a faithful expression pattern, even though the intronic enhancer alone directs an abnormally broad expression pattern. We present different models for "enhancer synergy,"whereby two enhancers with overlapping activities produce authentic patterns of gene expression.