Altering enhancer-promoter linear distance impacts promoter competition in cis and in trans.

In Drosophila, pairing of maternal and paternal homologs can permit trans-interactions between enhancers on one homolog and promoters on another, an example of a phenomenon called transvection. When chromosomes are paired, promoters in cis and in trans to an enhancer can compete for the enhancer's activity, but the parameters that govern this competition are as yet poorly understood. To assess how the linear spacing between an enhancer and promoter can influence promoter competition in Drosophila, we employed transgenic constructs wherein the eye-specific enhancer GMR is placed at varying distances from a heterologous hsp70 promoter driving a fluorescent reporter. While GMR activates the reporter to a high degree when the enhancer and promoter are spaced by a few hundred base pairs, activation is strongly attenuated when the enhancer is moved 3 kb away. By examining transcription of endogenous genes near the point of transgene insertion, we show that linear spacing of 3 kb between GMR and the hsp70 promoter results in elevated transcription of neighboring promoters, suggesting a loss of specificity between the enhancer and its intended transgenic target promoter. Furthermore, increasing spacing between GMR and hsp70 by just 100 bp can enhance transvection, resulting in increased activation of a promoter on a paired homolog at the expense of a promoter in cis to the enhancer. Finally, cis-/trans-promoter competition assays in which one promoter carries mutations to key core promoter elements show that GMR will skew its activity toward a wild-type promoter, suggesting that an enhancer is in a balanced competition between its potential target promoters in cis and in trans.