NUCLEOSOMES AND REGULATION OF GENE-EXPRESSION - STRUCTURE OF THE HIV-1 5'LTR

Packaging of DNA into chromatin adds complexity to the problem of regu lation of gene expression. Nucleosomes affect the accessibility of tra nscription factors to occupy their binding sites in chromatin of eukar yotic cells. The disruption of nucleosome structure within the enhance r/promoter region of the integrated HIV-1 proviral genome is an instru ctive example of a chromatin remodeling process during transcrip tiona l activation. To investigate the mechanism responsible for generating nuclease hypersensitive sites that exist in vivo in the promoter/enhan cer region of the 5'LTR (long terminal repeat) of integrated HIV-1 we have utilized an in vitro chromatin assembly system with Xenopus oocyt e extracts. Chromatin assembly in the presence of Sp1 and NF kappa B t ranscription factors induces DNase I hypersensitive sites on either si de of their binding sites and positions the adjacent nucleosomes. This structure can also be formed in a factor-induced, ATP-dependent chrom atin remodeling process and closely resembles the in vivo chromatin st ructure. The DNase I hypersensitive sites that form within the HIV LTR are probably histone-free and remain after removal of transcription f actors.