Promoter strength delimits enhancer threshold in the early Drosophila embryo

The enhancer threshold is defined as the minimum concentration of transcription factors (TFs) required to elicit an enhancer response in a given time and space. Here, evidence is presented that the enhancer threshold is relative to promoter strength in the early <i>Drosophila</i> embryo. The apparently inactive <i>even-skipped</i> (<i>eve</i>) minimal stripe element (MSE), in which a single Hunchback (Hb)-binding site is deleted, is functionally complemented by the <i>hsp70</i> promoter in transgenic embryos. Forced pause release of RNA polymerase II (Pol II) and transcription bubble assays show that both <i>eve</i> and <i>heat shock protein 70</i> (<i>hsp70</i>) promoters exhibit paused Pol II. However, bioinformatics analyses and transient transfection assays indicate that the strength of the <i>hsp70</i> promoter is much stronger than that of the <i>eve</i> promoter. Consistently, inactive MSE function is also restored by promoters stronger than the <i>eve</i> promoter. It is conceivable that the functional complementarity between enhancer and promoter strengths defines the enhancer threshold, thus determining whether a genomic locus acts as an enhancer for a particular promoter.