DIFFERENCES BETWEEN MYOD DNA-BINDING AND ACTIVATION SITE REQUIREMENTSREVEALED BY FUNCTIONAL RANDOM SEQUENCE SELECTION

A method has been developed for selecting functional enhancer/promoter sites from random DNA sequences in higher eukaryotic cells. Of sequen ces that were thus selected for transcriptional activation by the musc le-specific basic helix-loop-helix protein MyoD, only a subset are sim ilar to the preferred in vitro binding consensus, and in the same prom oter context an optimal in vitro binding site was inactive. Other sequ ences with full transcriptional activity instead exhibit sequence pref erences that, remarkably, are generally either identical or very simil ar to those found in naturally occurring muscle-specific promoters. Th is first systematic examination of the relation between DNA binding an d transcriptional activation by basic helix-loop-helix proteins indica tes that binding per se is necessary but not sufficient for transcript ional activation by MyoD and implies a requirement for other DNA seque nce-dependent interactions or conformations at its binding site.