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The precise positioning of nucleosomes plays a critical role in the regulation of gene expression by modulating the DNA binding activity of trans-acting factors. However, molecular determinants responsible for positioning are not well understood. We examined whether the removal of the core histone tail domains from nucleosomes reconstituted with specific DNA fragments led to alteration of translational positions. Remarkably, we find that removal of tail domains from a nucleosome assembled on a DNA fragment containing a Xenopus borealis somatic-type 5S RNA gene results in repositioning of nucleosomes along the DNA, including two related major translational positions that move about 20 bp further upstream with respect to the 5S gene. In a nucleosome reconstituted with a DNA fragment containing the promoter of a Drosophila alcohol dehydrogenase gene, several translational positions shifted by about 10 bp along the DNA upon tail removal. However, the positions of nucleosomes assembled with a DNA fragment known to have one of the highest binding affinities for core histone proteins in the mouse genome were not altered by removal of core histone tail domains. Our data support the notion that the basic tail domains bind to nucleosomal DNA and influence the selection of the translational position of nucleosomes and that once tails are removed movement between translational positions occurs in a facile manner on some sequences. However, the effect of the N-terminal tails on the positioning and movement of a nucleosome appears to be dependent on the DNA sequence such that the contribution of the tails can be masked by very high affinity DNA sequences. Our results suggest a mechanism whereby sequence-dependent nucleosome positioning can be specifically altered by regulated changes in histone tail-DNA interactions in chromatin.

The core histone tail domains are known to be key regulators of chromatin structure and function. The tails are required for condensation of nucleosome arrays into secondary and tertiary chromatin structures, yet little is known regarding tail structures or sites of tail interactions in chromatin. We have developed a system to test the hypothesis that the tails participate in internucleosomal interactions during salt-dependent chromatin condensation, and here we used it to examine interactions of the H3 tail domain. We found that the H3 tail participates primarily in intranucleosome interactions when the nucleosome array exists in an extended "beads-on-a-string" conformation and that tail interactions reorganize to engage in primarily internucleosome interactions as the array successively undergoes salt-dependent folding and oligomerization. These results indicated that the location and interactions of the H3 tail domain are dependent upon the degree of condensation of the nucleosomal array, suggesting a mechanism by which alterations in tail interactions may elaborate different structural and functional states of chromatin.

Reconstitution of a DNA fragment containing a 5S RNA gene from Xenopus borealis into a nucleosome greatly restricts binding of the primary 5S transcription factor, TFIIIA. Consistent with transcription experiments using reconstituted templates, removal of the histone tail domains stimulates TFIIIA binding to the 5S nucleosome greater than 100-fold. However, we show that tail removal increases the probability of 5S DNA unwrapping from the core histone surface by only approximately fivefold. Moreover, using site-specific histone-to-DNA cross-linking, we show that TFIIIA binding neither induces nor requires nucleosome movement. Binding studies with COOH-terminal deletion mutants of TFIIIA and 5S nucleosomes reconstituted with native and tailless core histones indicate that the core histone tail domains play a direct role in restricting the binding of TFIIIA. Deletion of only the COOH-terminal transcription activation domain dramatically stimulates TFIIIA binding to the native nucleosome, while further C-terminal deletions or removal of the tail domains does not lead to further increases in TFIIIA binding. We conclude that the unmodified core histone tail domains directly negatively influence TFIIIA binding to the nucleosome in a manner that requires the C-terminal transcription activation domain of TFIIIA. Our data suggest an additional mechanism by which the core histone tail domains regulate the binding of trans-acting factors in chromatin.

The core histone tail domains are key regulators of eukaryotic chromatin structure and function and alterations in the tail-directed folding of chromatin fibers and higher order structures are the probable outcome of much of the post-translational modifications occurring in these domains. The functions of the tail domains are likely to involve complex intra- and inter-nucleosomal histone-DNA interactions, yet little is known about either the structures or interactions of these domains. Here we introduce a method for examining inter-nucleosome interactions of the tail domains in a model dinucleosome and determine the propensity of each of the four N-terminal tail domains to mediate such interactions in this system. Using a strong nucleosome "positioning" sequence, we reconstituted a nucleosome containing a single histone site specifically modified with a photoinducible cross-linker within the histone tail domain, and a second nucleosome containing a radiolabeled DNA template. These two nucleosomes were then ligated together and cross-linking induced by brief UV irradiation under various solution conditions. After cross-linking, the two templates were again separated so that cross-linking representing inter-nucleosomal histone-DNA interactions could be unambiguously distinguished from intra-nucleosomal cross-links. Our results show that the N-terminal tails of H2A and H2B, but not of H3 and H4, make internucleosomal histone-DNA interactions within the dinucleosome. The relative extent of intra- to inter-nucleosome interactions was not strongly dependent on ionic strength. Additionally, we find that binding of a linker histone to the dinucleosome increased the association of the H3 and H4 tails with the linker DNA region.

We utilized a site-specific cross-linking technique to investigate the mechanism of nucleosome remodeling by hSWI/SNF. We found that a single cross-link between H2B and DNA virtually eliminates the accumulation of stably remodeled species as measured by restriction enzyme accessibility assays. However, cross-linking the histone octamer to nucleosomal DNA does not inhibit remodeling as monitored by DNase I digestion assays. Importantly, we found that the restriction enzyme-accessible species can be efficiently cross-linked after remodeling and that the accessible state does not require continued ATP hydrolysis. These results imply that the generation of stable remodeled states requires at least transient disruption of histone-DNA interactions throughout the nucleosome, while hSWI/SNF-catalyzed disruption of just local histone-DNA interactions yields less-stable remodeled states that still display an altered DNase I cleavage pattern. The implications of these results for models of the mechanism of SWI/SNF-catalyzed nucleosome remodeling are discussed.