Exposure of histone F1 subfractions in chromatin

Accessibility of the globular domain of histones H1 and H5 to antibodies upon folding of chromatin

Antibodies to the globular domain of histones H1 and H5 were purified by affinity chromatography and used to study the accessibility of this region of H1 and H5 in folded and unfolded rat liver and hen erythrocyte chromatin respectively. The different conformations of the chromatin filament were induced by varying the ionic strength from 1 mM to 80 mM NaCl and maintained by fixation with glutaraldehyde. Treatment with glutaraldehyde at a given salt concentration affected neither the orientation of nucleosomes relative to the fiber axis nor the compactness of chromatin. Solid-phase immunoassay and inhibition experiments showed no binding of the antibody against the globular domain of H1 to chromatin at the entire range of salt concentrations, while the antibody to the whole H1 molecule reacted with chromatin at low salt. On the other hand, the antibody to the globular region of H5 reacted with hen erythrocyte chromatin independently of the extent of chromatin condensation. These results indicate that the antigenic determinants of the globular domain of H5 are accessible to the antibody both in folded and unfolded chromatin, while those of the same region of H1 are masked, probably by interaction with DNA or proteins.

Specificity studies on anti-histone H1 antibodies obtained by different immunization methods

Antibodies were elicited against chromatographically purified histone H1 subfractions or against their complexes with RNA and their specificity studied by enzyme-linked immunosorbent assay. The results show that complexing of the pure protein with RNA does not lead to any significant increase in the antibody titer and results in obtaining antibodies predominantly against the common antigenic determinants present in the H1 histone class. On the other hand, using pure histone fractions for immunization gives rise to antibody populations reacting mainly with the subfraction-specific determinants on the histone molecule. In view of these results the literature data should be interpreted with caution.

Roles of H1 domains in determining higher order chromatin structure and H1 location

Peptides derived from calf thymus H1 and rat liver H1, comprising only the globular and COOH-terminal domains of the intact molecule and therefore lacking NH2-terminal domains, have been shown by reconstitution to be as effective as the complete H1 molecule in inducing higher-order-chromatin structure. As the globular domain of H1 alone cannot induce chromatin folding, our results demonstrate that this function is primarily controlled by the COOH-terminal domain of the molecule. Surprisingly, these peptides do not locate correctly with respect to the nucleosome. This is demonstrated by their failure to confer upon reconstitutes the ability to protect DNA fragments of chromatosome length when digested with micrococcal nuclease. The precise placement of the H1 molecule (globular domain) with respect to the nucleosome is shown to be influenced by the "tail" domains of both H1 and the core histones.

Distribution of antigenicity in chicken erythrocyte histone H5

We have compared the inhibitory strengths of eight peptides from chicken histone H5 to that of the parent protein (189 residues) in complement fixation by guinea pig anti-H5 serum complexed with the homologous histone. Two precisely delineated regions (residues 59 to 65 and 94 to 99) and two less-defined regions (between 66 and 93; 100 and 189) are depicted. The sequences of particular consequences are quite conserved in avian histone H5 while the most variable N-terminal portion of 31 residues has no inhibitory effect.

Immunofluorescent study of histone H5 in chick erythroid cells from developing embryos and adults

Indirect immunofluorescence has been used to detect the presence of histone H5 in single cells during avian erythropoiesis. Evidence is presented which demonstrates that this method specifically detects histone H5. The results show that the increase in the amount of H5 which can be extracted from nuclei during erythropoiesis is accounted for both by an increase in the amount of H5 per cell and by an increase in the total number of cells containing histone H5. Of particular interest are the observations that histone H5 is arranged within the nucleus in several distinct, reproducible patterns and that different patterns predominate at different stages of erythroid cell development. In addition to suggesting new possibilities for the role of histone H5, these findings indicate that immunofluorescence may be used profitably to explore the supramolecular organization of chromatin.

Chromatin structure visualization by immunoelectron microscopy

Antibodies elicited in rabbits by chromatin and by purified histone H2B have been used to study the structure of chromatin by immunoelectron microscopy. Chromatin spread on grids reveals a structure of closely packed spherical particles with an average diameter of 104 A, arranged either in clusters or in linear arrays of beads, some of which have a supercoil-like arrangement. No DNA strings connecting the beads could be observed. Upon antibody binding, the diameter of the particles increases up to 300 A. This size is compatible with a model where one layer of gamma globulin molecules 110 A long encircles a sphere of chromatin 100 A in diameter. The presence of rabbit gamma globulins on the enlarged beads has been verified by the addition of ferritin-labeled goat anti-rabbit gamma globulins. Anti-chromatin sera which react with nonhistone proteins but not with free histones or DNA react with more than 95% of the beads; this suggests that most of the beads contain nonhistone proteins. Since the number of nonhistone proteins is large, it is improbable that each sphere contains a full complement of these proteins. We therefore suggest that the various chromatin spheres contain different types of nonhistone proteins. About 90% of the chromatin spheres reacted with antibodies to histone H2B, suggesting the most of the chromatin beads contain this type of histone.