THE ESTABLISHMENT OF ACTIVE PROMOTERS IN CHROMATIN

The organization of eukaryotic genomes as chromatin provides the frame work within which regulated transcription occurs in the nucleus. The a ssociation of DNA with chromatin proteins required to package the geno me into the nucleus is, in general, inhibitory to transcription, and t herefore provides opportunities for regulated transcriptional activati on. Granting access to the cis-acting elements in DNA, a prerequisite for any further action of the trans-acting factors involved, requires the establishment of local heterogeneity of chromatin and, in some cas es, extensive remodeling of nucleosomal structures. Challenging proble ms relate to the establishment of this heterogeneity at the level of t he single nucleosome and to the mechanisms that operate when nucleosom al arrays are reorganized. Recent developments indicate that chromatin reconstitution in cell-free systems allows the biochemical analysis o f the interplay between transcription factors and chromatin components that brings about regulated-transcription.