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Using a combination of small-angle x-ray scattering and fluorescence resonance energy transfer (FRET) measurements we have determined the role of the H3 and H4 histone tails independently in stabilizing nucleosomes and the extent to which the terminal ends of DNA unwrap from the nucleosome core. We have reconstructed solution scattering envelopes for recombinant wild-type, H3 and H4 tail-removed mutants and fit all scattering data with predictions from PDB models. Based on these data combined with FRET results, we found that while all nucleosomes exhibited DNA unwrapping, the extent of this unwrapping was increased for nucleosomes with the H3 tails removed but decreased in nucleosomes with the H4 tails removed. Studies of salt concentration effects showed small (compared to tail effects) but noticeable effects as a function of monovalent ions. These global and local structural data are …