Enhancer elements share local homologous twist-angle variations with a helical periodicity

Several experiments have shown that some enhancers can be exchanged between different genomes. The transferred enhancers were functional (cf. Levinson, B., Khoury, G., Vande Woude, G. and Gruss, P. (1982) Nature 295, 568-572). This argues that these exchanged fragments are recognized as enhancers and possess some common characteristics which other sequences lack. Extensive comparisons of enhancers yielded only very limited nucleotide sequence homology, which appears to be insufficient for enhancer recognition. We suggest that the enhancers located and sequenced to date have recurring, periodic homologous twist-angle (t_g) patterns. This helical periodicity and the symmetric nature of the repeating twist-angle features present a recurring spatial geometry. It also offers a possible explanation of the fact that inverted enhancers are still functional. Regions of large twist, roll or main-chain torsion angle δ deviations from regular B-DNA may facilitate enhancer recognition especially when distant from promoter elements. Tissue specificity may be encoded in additional sequence or structural features.