Modulation of eukaryotic gene expression is influenced by the organization of regulatory DNA-elements in chromatin. The mouse mammary tumor virus (MMTV) promoter exhibits regularly positioned nucleosomes that reduce the accessibility of the binding sites for sequence-specific transcription factors, in particular nuclear factor (NF1). Hormonal induction of the MMTV promoter is accompanied by remodeling of the nucleosomal 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 compared 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 for particles containing only the H3/H4 tetramer, showing that the absence of H2A/H2B dimers does not alter positioning. The rotational orientation of the DNA double helix in both types of particles is essentially identical. However, the ends of the nucleosomal DNA as well as its central region are more accessible to cleavage reagents in the tetramer particle than in the octamer particle. In agreement with these structural features, the transcription factors NF1 and OTF1 were able to bind to their cognate sites on the tetramer particle, while they could not gain access to the same sites on the surface of the octamer particle. The DNase I digestion pattern of octamers treated with partially purified SWI/SNF complex from HeLa cells in the presence of ATP is indistinguishable from that of tetramer particles, suggesting that the SWI/SNF complex promotes ATP-dependent remodeling of the octamer particle but not of tetramer particles. These results are compatible with a hormone-induced removal of histone H2A/H2B during MMTV induction.