A cis-acting regulatory sequence that markedly increases expression of a neighbouring gene. Enhancers are typically capable of operating over considerable distances (sometimes ~50 kb) upstream or downstream of the gene, and in either orientation. SilencerA cis-acting regulatory sequence that decreases expression of a neighbouring gene. Upstream activating sequenceA cis-acting regulatory sequence in yeast that is distinct from the promoter and increases expression of a neighbouring gene.

No abstract

Methylation of histones at specific residues plays an important role in transcriptional regulation. Chromatin immunoprecipitation of dimethylated lysine 9 on histone H3 across 53 kilobases of the chicken beta-globin locus during erythropoiesis shows an almost complete anticorrelation between regions of elevated lysine 9 methylation and acetylation. Lysine 9 is methylated most over constitutive condensed chromatin and developmentally inactive globin genes. In contrast, lysine 4 methylation of histone H3 correlates with H3 acetylation. These results lead us to propose a mechanism by which the insulator in the beta-globin locus can protect the globin genes from being silenced by adjacent condensed chromatin.

The 1.2-kb DNA sequence element (5HS4) at the 5 end of the chicken ␤-globin locus has the two defining properties of an insulator: it prevents an ''external'' enhancer from acting on a promoter when placed between them (''enhancer blocking'') and acts as a barrier to chromosomal position effect (CPE) when it surrounds a stably integrated reporter. We previously reported that a single CTCF-binding site in 5HS4 is necessary and sufficient for enhancer blocking. We show here that a 250-bp ''core'' element from within 5HS4 is sufficient to confer protection against silencing of transgenes caused by CPE. Further dissection of the core reveals that 5HS4 is a compound element in which it is possible to separate enhancer blocking and barrier activities. We demonstrate that full protection against CPE is conferred by mutant 5HS4 sequences from which the CTCF-binding site has been deleted. In contrast, mutations of four other protein binding sites within 5HS4 result in varying reductions in the ability to protect against CPE. We find that binding sites for CTCF are neither necessary nor sufficient for protection against CPE. Comparison of the properties of 5HS4 with those of other CTCF-binding enhancer-blocking elements suggests that CPE protection is associated with maintenance of a high level of histone acetylation near the insulator, conferred by insulator binding-proteins other than CTCF.CTCF ͉ chromatin domain ͉ boundaries I nsulators are DNA sequence elements that protect transcribed regions from outside regulatory influences. They are present near chromatin domain boundaries or at sites where they prevent inappropriate activation of a promoter by a nearby heterologous enhancer. A growing number of insulators with varied binding sequences and associated proteins have been found in Drosophila, and a few have been found in vertebrates (1-3). Some time ago we identified, at the 5Ј end of the chicken ␤-globin locus, an element (5ЈHS4) with the characteristic properties of an insulator (4). We applied two defining tests for insulator activity. In the first, the ''enhancer-blocking'' assay, we placed a 1.2-kb DNA sequence element containing 5ЈHS4 between an enhancer and promoter and showed that the action of the enhancer was impeded (4-6). This impedance did not occur when the insulator was inserted elsewhere. In the second assay, we showed that when double copies of this 1.2-kb sequence were placed on either side of an erythroid-specific reporter and stably integrated into an erythroid cell line, the expression of the reporter was uniform from one line to another (7). Expression was maintained after 80 days of incubation in the absence of selection. In contrast, expression was variable among uninsulated lines, and in most cases expression of uninsulated lines was extinguished in far less than 80 days. We concluded that this variability of expression and rapid extinction were manifestations of chromosomal position effects arising from the influence on the reporter of dominant regulatory elements flanking the sites of inte...