No abstract

The arginine-rich histone, H3, isolated from avian erythrocytes, can dimerize by forming a disulfide linkage between the single cysteine sulfhydryl residues at position 110 of the H3 polypeptide chain. The H3 dimer can be substituted for undimerized H3 in experiments in which the nucleosome is reconstituted from DNA and mixtures of the four "core" histones, H2A, H2B, H3, and 114. We report here that reconstituted nucleosomes containing H3 dimer are indistinguishable, by a number of criteria, either from native nucleosomes or from reconstitutes containing H3 monomer. The criteria include the pattern of susceptibility of the complex to nucleases, the amount of DNA supercoiling induced by histone binding, and the hydrodynamic properties of reconstituted nucleosome "core" preparations.The results suggest that the residues in the neighborhood of position 110 on each H3 molecule are in close contact in the nucleosome. If, as has been proposed, the nucleosome has a dyad axis, then the disulfide bridge between H3 molecules must lie on this axis.An understanding of the biological activity of chromatin will require detailed knowledge of the internal architecture of its fundamental repeating subunit, the nucleosome.Considerable evidence has accumulated that the DNA in the 140-base-pair nucleosome "core" is wrapped around (1, 2) a histone octamer consisting of two of each of the core histones H2A, H2B, H3, and H4 (3). Studies of histone-DNA interactions have shown that the arginine-rich histones, H3 and H4, play a central role in nucleosome organization (4-10). We are therefore interested in obtaining further information about the way in which these histones interact with each other within the nucleosome. An obvious approach is to use chemical crosslinking reagents to estimate the proximity of functional groups in the proteins; there have been many such studies of histone interactions in chromatin (3). However, most crosslinking reagents can react at more than one site, and so far it has not been possible to identify the specific amino acid residues within the histones that are involved in the crosslinking.There is one crosslinking reaction that may avoid some of these problems: the formation of intermolecular disulfide bonds between cysteine residues. This is a particularly useful reaction for the study of histones because H3 is the only histone that has cysteine and in organisms other than higher mammals there is only one such residue per molecule (11)(12)(13)(14). The cysteine at this one position, residue 110 of H3, has been preserved, with the single exception of yeast H3 (15), throughout evolution.It is well known (11,12,16) that H3 molecules can dimerize in vitro through a disulfide bond between the cysteine residues at position 110. We report here that such a dimer is capable of entering into the formation of nucleosomes. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §...

BackgroundThe protein encoded by the gene ybgI was chosen as a target for a structural genomics project emphasizing the relation of protein structure to function.ResultsThe structure of the ybgI protein is a toroid composed of six polypeptide chains forming a trimer of dimers. Each polypeptide chain binds two metal ions on the inside of the toroid.ConclusionThe toroidal structure is comparable to that of some proteins that are involved in DNA metabolism. The di-nuclear metal site could imply that the specific function of this protein is as a hydrolase-oxidase enzyme.