To understand this post, you're going to need to know a little bit about gene regulation. The location on the DNA where the a gene's messenger RNA starts is called a promoter—it contains binding sites for proteins that help start the RNA-making process. For many genes, that's all that's needed. But for those with complex regulatory control—and that includes most of the genes involved in embryonic development—other sequences are needed to ensure that a promoter is only active in the right tissues at the right time. These sequences, called enhancers, can be more or less anywhere, even hundreds of kilobases distant from the promoter they regulate.
This raises a couple of obvious questions: how do they ever find the promoter? If they can work at such large distances, why don't the enhancers just activate all the genes nearby? To give you a sense of the scale of the problem, I'll draw an analogy based on the Bithorax gene complex that's the subject of a paper from the most recent issue of Development. Two genes and an enhancer (among other things) reside in a region that's the DNA equivalent of over 100 miles long. The promoters of the genes are only about 100 feet long. Somehow, the enhancer not only finds a promoter to regulate, but it find the right one.
The new paper helps describe how this happens. It turns out that the promoter has a sequence just next to it that helps specifically attract the enhancer to that promoter—the researchers called it a tether. Delete the tether, and the enhancer will regulate whichever gene is closest. Add the tether to an unrelated gene, and the enhancer will regulate that. You can even stick other genes or DNA insulating sequences between the enhancer and its tether, and the enhancer will ignore them all and work with the promoter next to the tether. To draw another analogy, the tether acts like a postal code to help the enhancer find the right neighborhood.
The authors cite an earlier paper that found something similar in a completely different complex of genes and regulatory elements, suggesting that tethering represents a general mechanism for gene regulation. With dozens of fly genomes now available, it's possible that a few more examples of this will be enough to let bioinformatics gurus fish out tethering sequences, so that the biochemists can tell us how they work.
Development, 2007. DOI: 10.1242/dev.010744