Histone H1-DNA interactions and their relation to chromatin structure and function

DNA binding of histone H1 is modulated by nucleotides

Histone H1 acts as a general repressor of transcription in eukaryotes by organizing nucleosomes into inaccessible condensed forms of chromatin. The capability of H1 to bind to DNA with some sequence specificity is likely to be critical in the control of these processes. We show here that ATP and several other nucleotides, including non-hydrolyzable derivatives, can inhibit DNA binding of H1. The results also show that ATP differentially affects binding of H1 to DNA in a fashion enhancing nucleotide sequence specificity of the binding. The study suggests a novel mechanism of modulation of H1 activity that has important implications for the role of H1 as a transcriptional regulator.

The Ku complex is modulated in response to viral infection and other cellular changes

The complex of Ku with DNA is demonstrated to have multiple forms as assayed by gel retardation analysis. In CV1 cells this variation of complex can be modulated in response to viral infection with SV40. By Western blot analysis, a correlation can be made between modification of the complex formed on DNA in response to viral infection with variation of the 85 kDa subunit of Ku. Modification of the 85 kDa subunit can also be seen when cells are exposed to various extracellular stimuli including variation in serum levels, PMA and CaPO4.

Chromatin changes during the cell cycle

Considerable progress has recently been made in elucidating the biochemical mechanisms regulating changes in chromatin structure during all stages of the cell cycle. Although anticipated, the apparently ubiquitous role played by phosphorylation/dephosphorylation reactions in modulating these changes is, nonetheless, remarkable.

Histone H1 interacts specifically with certain regions of the mouse alpha-globin gene

We used fragments of a cloned mouse alpha-globin gene to determine if histone H1 interacts selectively with defined regions of a eukaryotic gene. The use of intact plasmids instead of isolated fragments permitted study of relevant sequences in their superhelical form. Several independent experimental approaches (filter binding, precipitation, binding to protein immobilized on nitrocellulose membranes, and agarose gel electrophoresis of the protein-DNA complexes) were used and the histone-DNA interaction was investigated under both noncompetitive and competitive conditions. Binding to subclones encompassing the 5' end of the gene and the first half of the coding sequence is preferred over binding to other subclones. The expression of the sequence-specific selectivity depends on the ionic strength of the binding reaction; the selectivity is mainly expressed under conditions of non-cooperative binding of the histone to DNA. No correlation is observed between AT content and relative affinity of binding to H1. Evidently, other features of DNA structure are involved in the specific H1 binding.

Nucleoside triphosphate binding and hydrolysis by histone H1

We present here further evidence supporting that histone H1 contains a nucleotide binding site interacting e.g. with ADP, ATP, GDP and GTP. The finding is in accordance with the previous observation that nucleotides modulate recognition of DNA by H1. Most interestingly, H1 appears to be capable of hydrolyzing NTPs and incorporating phosphate to exogenous proteins. The mode of nucleotide action on H1 may be considered highly analogous to that of GTPases. Nuclear receptors may thus act through mechanisms similar to those for receptors on the plasma membrane.

Arabidopsis thaliana H1 histones. Analysis of two members of a small gene family

We have isolated two Arabidopsis thaliana cDNA clones that encodes different H1 histone proteins. The H1-1 and H1-2 proteins are 274 and 273 amino acids in length, respectively. Unlike the H1 histones within a single animal species, the two plant H1 proteins share little sequence similarity outside the protein's central globular domain. Within the globular domain, a pentapeptide that is extremely well conserved in animal H1 histones, is not found in either of the plant proteins. Southern blot analysis suggests that A. thaliana has only three H1 histone genes. A genomic clone encoding the H1-1 protein was isolated and the protein-coding region was found to consist of two exons separated by a 104-bp intron. The site of transcriptional initiation of the H1-1 gene was mapped by primer-extension analysis and a conserved octamer motif, identical to that observed in most plant core histone genes that have been characterized to date, was found 101 nucleotides upstream of the presumed transcription-initiation site. The 3' portion of the gene encoding H1-2 was also isolated and sequenced. When the 3'-flanking regions of the two H1 genes were compared, several highly conserved sequences were observed that might be convergently transcribed relative to the histone genes.