THE MOUSE MAMMARY-TUMOR VIRUS PROMOTER POSITIONED ON A TETRAMER OF HISTONES H3 AND H4 BINDS NUCLEAR FACTOR-1 AND OTF1

Modulation of eukaryotic gene expression is influenced by the organiza tion of regulatory DNA-elements in chromatin. The mouse mammary tumor virus (MMTV) promoter exhibits regularly positioned nucleosomes that r educe the accessibility of the binding sites for sequence-specific tra nscription factors, in particular nuclear factor (NF1). Hormonal induc tion of the MMTV promoter is accompanied by remodeling of the nucleoso mal structure, but the biochemical nature of these structural changes is unknown. Using recombinant histones, we have now assembled the MMTV promoter in particles containing either an octamer of the histones H3 , H4, H2A and H2B or a tetramer of histones H3 and H4, and have compar ed the two particles in terms of structure, positioning, and exclusion of transcription factors. Using site-directed hydroxy radicals to map histone locations, two main nucleosome positions are found with dyads at position -107 and at -127. The same two main positions are found f or particles containing only the H3/H4 tetramer, showing that the abse nce of H2A/H2B dimers does not alter positioning. The rotational orien tation of the DNA double helix in both types of particles is essential ly identical. However, the ends of the nucleosomal DNA as well as its central region are more accessible to cleavage reagents in the tetrame r particle than in the octamer particle. in agreement with these struc tural features, the transcription factors NF1 and OTF1 were able to bi nd to their cognate sites on the tetramer particle, while they could n ot gain access to the same sites on the surface of the octamer particl e. The DNase I digestion pattern of octamers treated with partially pu rified SWI/SNF complex from HeLa cells in the presence of Am is indist inguishable from that of tetramer particles, suggesting that the SWI/S NF complex promotes ATP-dependent remodeling of the octamer particle b ut not of tetramer particles. These results are compatible with a horm one-induced removal of histone H2A/H2B during MMTV induction. (C) 1998 Academic Press Limited.