A hypothesis for chromatin domain opening

The eukaryotic genome is organized into different domains by cis-acting elements, such as boundaries/insulators and matrix attachment regions, and is packaged with different degrees of condensation. In the M phase, the chromatin becomes further highly condensed into chromosomes. The first step for transcriptional activation of a given gene, at a particular time during development, in any locus, is the opening of its chromatin domain. This locus needs to be kept in this state in each early G(1) phase during every cell cycle. Certain distal enhance elements, including locus control regions (LCRs) and enhancers, are believed to perform this target chromatin domain opening process and several models have been proposed to explain distal enhance action. But they did not explain precisely how a given chromatin domain is opened. Based on various studies, we propose a hypothesis for the mechanism of opening chromatin on a large scale. One important mechanism may involved breaking one or two DNA strands and reducing the linking numbers within chromatin domain. The topological changes can overpass some complexes formed on DNA strands and can be transmitted from specific localized points over a broad region, until boundary elements or insulators are reached. These may initiate downstream events such as propagation of histone acetylation and the binding of transcription factors to proximal promoters and may further augment the action mediated by distal enhancer elements. (C) 2003 Wiley Periodicals, Inc.