Putting Boundaries on Silence

RAP1 and core histones can spread from the telomere at a terminal truncation into adjacent regions (reviewed Department of Biology St. Louis, Missouri 63130 by Grunstein, 1998). One can infer that discrete boundaries are needed to maintain the mosaic pattern of silent and active domains found under normal conditions; indeed , such " barriers " to the spread of heterochromatin The eukaryotic genome is enormously complex. Experi-and heterochromatin-like structure have recently been mental observations indicate that in addition to the sub-identified (see Table 1) in yeast and in chicken. division of the genome into distinct chromosomes, each In addition, the promiscuous behavior of many regula-chromosome is partitioned into distinct structural and tory elements also requires that the genome be parti-functional domains. This implies the presence of bound-tioned into discrete regulatory domains. Enhancers are aries, DNA elements that mark the border between adja-known to function over large distances, and have the cent domains, allowing them to maintain different func-potential to affect many different promoters. Some reso-tional states. Several different assays have been used to lution of the problem of enhancer promiscuity has come identify and describe boundaries. Elements have been with the recognition of a class of DNA elements that detected that can block enhancer function; these are serve as " insulators " of enhancer function (see Table 1). commonly referred to as " insulators. " Other elements The defining members of the group, scs and scsЈ, flank have been described within the Bithorax complex (BX-C) two hsp70 genes of Drosophila. These elements can of Drosophila melanogaster that delimit adjacent cis-block communication between an enhancer and a pro-regulatory domains. Recently, boundary elements have moter if they are positioned between them. Insulator been identified that appear to limit the spread of hetero-activity, so defined, has now been associated with a chromatin. The identification and characterization of number of sequence elements in Drosophila and higher these latter elements, which we will refer to as " barriers, " eukaryotes (Geyer, 1997; Bell and Felsenfeld, 1999). will be the major focus of this minireview. While some Another type of boundary element has been identified elements have both insulator and barrier activity, there within the Bithorax complex (BX-C) of Drosophila. The are suggestions that these functions may be separable BX-C complex contains a tandem array of regulatory in other cases. Further analysis will be needed to discern elements that control the expression of the Abd-B gene …