RESUMO
Although positioned nucleosomes are known to play a direct, localised role in regulating access to DNA sequence, they also have the potential, through their long-range distribution, to affect the detailed structure of the higher-order chromatin fibre. To investigate this possibility, we firstly mapped, in vitro, the sequence-dependent positions that the core histone octamer adopts when reconstituted onto DNA containing the ovine beta-lactoglobulin gene. These positioning sites are discussed in terms of their relative affinity for the histone octamer, their locations with respect to the gene sequence and their periodic distribution throughout the gene region. Secondly, we mapped, in vivo, the sites that nucleosomes occupy on the same sequence in liver nuclei, where the gene is transcriptionally inactive. Although the sequence is largely packaged into regularly spaced nucleosomes, reflecting a fibre of uniform higher-order structure, this organisation is disrupted by a number of unusual chromatin structures in a region stretching from the second to the third introns of the gene. A comparison of the in vitro and in vivo nucleosome positioning data shows that they are qualitatively and quantitatively related, suggesting that the structure of the higher-order chromatin fibre containing the beta-lactoglobulin gene is determined, in part, by the long-range organisation of the non-coding sequences within which the gene is embedded.
