PERIODICITY OF STRONG NUCLEOSOME POSITIONING SITES AROUND THE CHICKENADULT BETA-GLOBIN GENE MAY ENCODE REGULARLY SPACED CHROMATIN

Positioned nucleosomes contribute to both the structure and the functi on of the chromatin fiber and can play a decisive role in controlling gene expression, We have mapped, at high resolution, the translational positions adopted by limiting amounts of core histone octamers recons tituted onto 4.4 kb of DNA comprising the entire chicken adult beta-gl obin gene, its enhancer, and flanking sequences, The octamer displays extensive variation in its affinity for different positioning sites, t he range exhibited being about 2 orders of magnitude greater than that of the initial binding of the octamer, Strong positioning sites are l ocated 5' and 3' of the globin gene and in the second intron but are a bsent from the coding regions, These sites exhibit a periodicity (appr oximate to 200 bp) similar to the average spacing of nucleosomes on th e inactive beta-globin gene in vivo, which could indicate their involv ement in packaging the gene into higher-order chromatin structure. Ove rlapping, alternative octamer positioning sites commonly exhibit spaci ngs of 20 and 40 bp, but not of 10 bp, These short-range periodicities could reflect features of the core particle structure contributing to the pronounced sequence-depdendent manner in which the core histone o ctamer interacts with DNA.