Biology of the histones

The biological roles of the histones are multiple; by complexing to DNA they cause such DNA to be inactive as a template for RNA polymerase; they cause supercoiling of the DNA which would appear to be a fundamental requirement for further orders of supercoiling, presumable exemplified by metaphase chromosomes; a particular histone even forms interpolypeptide disulfide bridges during metaphase, apparently stabilizing the chromosomes in a highly condensed state. It is no doubt because of this multiplicity of functions that the primary structure of the individual histone species has been conserved to an extraordinary degree since the time of the common ancestor of the higher eukaryotes.     

Core histones and linker histones are imported into the nucleus by different pathways

Histones are the major structural proteins in eukaryotic chromosomes. This group of small very basic proteins consists of the H1 linker histones and the core histones H2A, H2B, H3 and H4. Despite their small size, the nuclear import of histones occurs by an active transport mechanism and not simply by diffusion. Histones contain several nuclear localisation signals (NLS) that can be subdivided into two different types of signal structures. We have previously shown that H1 histones are transported by a heterodimeric import receptor complex consisting of importin beta and importin 7, and we now describe the receptors required for the import of the core histones. Competition experiments using the in vitro transport assay indicate that the import pathway of the core histones differs from that of the linker histones and of nuclear proteins with classical NLS. In vitro binding assays show that each of the import receptors importin beta, importin 5, importin 7 and transportin, has the capacity to bind to any of the four core histones. Reconstitution experiments with recombinant factors indicate that each of these factors can independently serve as an import receptor for each of the core histones.     

Trout-liver histones

1. The histones of trout liver were examined and four main fractions (f1, f2b, f2a and f3) were isolated and characterized. 2. The amino acid analyses, N-terminal group analyses and starch-gel electrophoresis patterns are remarkably similar to the corresponding fractions of calf thymus. 3. The group f2a was also separated into two subfractions, f2al and f2a2, which are similar to those of calf thymus.     

Electrophoretic study of plant histones: Comparison with vertebrate histones

The histones of seven plant species (barley, leek, onion, pea, radish, rye, and wheat) were isolated and compared to the histones of calf thymus and rat liver using electrophoresis on polyacrylamide-urea and polyacrylamide-SDS gels. It was found that the F1 histone of plants contains more subspecies and has generally higher molecular weights than their animal histone counterparts. Histones F3 of plants and animals have identical molecular weights and similar but not identical mobilities on polyacrylamide-urea gels. No histones were found in plants which have molecular weights and mobilities on polyacrylamide-urea gels which resemble the values for histones F2a2 and F2b of animals, but instead the series of histones observed differ from any of the animal histones. These plant histones may represent either substantially modified forms of F2a2 and F2b, or else may be a different class of histone molecules unique to plants. Fractions F2al in plants and animals are identical in electrophoretic behavior, but seem to differ in degree of acetylation.     

Archaeal histones and the origin of the histone fold

Histone sequences have been identified in many archaeal genomes and in environmental samples, and they constitute a family of proteins that are structural homologs of the eukaryotic core histones. Most archaeal histones conform to the single histone-fold structural models that have been described, but a few histone variants exhibit short insertions, additional domains or fusions. Interpretation of these structural variations offers clues to the steps that might have occurred during the evolution and specialization of eukaryotic core histones.     

Sequential ADP-ribosylation pattern of nucleosomal histones. ADP-ribosylation of nucleosomal histones

The pattern of nucleosomal histones poly(ADP-ribosyl)ation is changed under conditions which affect the poly(ADP-ribosyl)ation state of the enzyme. At low NAD concentrations the enzyme can poly(ADP-ribosyl)ate histones H1 and H1, H2A, A2A, and H2B. However at NAD concentrations above 10 microM the enzyme preferentially poly(ADP-ribosyl)ates histone H1 to a hyper ADP-ribosylated form. Furthermore we have observed hyper ADP-ribosylation of histone H2B at NAD concentrations of 10 microM suggesting that histone H2B can undergo the same type of ADP-ribosylation pattern as histone H1. Also at higher NAD concentrations an elongation of the polymer attached to the enzyme and other nuclear proteins takes place.