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

Positioned nucleosomes contribute to both the structure and the function 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 reconstituted onto 4.4 kb of DNA comprising the entire chicken adult beta-globin gene, its enhancer, and flanking sequences, The octamer displays extensive variation in its affinity for different positioning sites, the range exhibited being about 2 orders of magnitude greater than that of the initial binding of the octamer, Strong positioning sites are located 5' and 3' of the globin gene and in the second intron but are absent from the coding regions, These sites exhibit a periodicity (approximate to 200 bp) similar to the average spacing of nucleosomes on the inactive beta-globin gene in vivo, which could indicate their involvement in packaging the gene into higher-order chromatin structure. Overlapping, alternative octamer positioning sites commonly exhibit spacings 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 octamer interacts with DNA.