Tethering elements

Different mechanisms have been proposed to explain the specificity of enhancer-promoter interactions, including promoter competition and insulator DNA elements. In the July 9 Proceedings of the National Academy of Sciences, Calhoun et al. propose a different model, involving promoter-proximal tethering elements (Proc Natl Acad Sci USA2001,99:9243-9247). They studied the Antennapediagene complex (ANT-C) of Drosophila that contains a cluster of homeobox genes. Expression of the Scr selector gene depends on a distal enhancer T1 located 25 kb upstream. In between Scr and T1 lies the pair-ruled gene ftz, which is regulated by the intergenic enhancer AE1. Calhoun et al. attached the Scr and ftz promoter regions to CAT and LacZ reporter genes; they then showed that the core promoter elements were not important for regulating enhancer specificity. Promoter deletion constructs demonstrated that an Scr promoter fragment of 450 bp was sufficient to direct T1 to an adjacent gene promoter. The authors propose that the Scr450 elements acts as a 'tether' for the distal T1 enhancer.