Distribution of tissue-specific tightly bound non-histone proteins in the first level of repeating chromatin structures

Histones and DNA methylation in mammalian chromatin. II. Presence of non-inhibitory tightly-bound histones

After removal, by high-salt extraction, of the loosely-bound components present in human placenta chromatin, tightly-bound cationic proteins could be solubilized, by acid extraction, from the 'stripped' chromatin, as well as from the 'stripped' loops or from the 'digested matrix'. These acid-soluble tightly-bound proteins are, in terms of apparent molecular mass and immunoreactivity, quite similar to the 'typical', loosely-bound histones, and, similarly to their 'loosely-bound' counterparts, they can be subdivided in distinct H1-, H2A-, H2B-, H3- and H4-like components, the 'digested matrix' being however characterized by the absence of tightly-bound H1. These tightly-bound histones, at variance from the 'typical' ones, readily find a right-handed helical conformation upon renaturation by progressive dialyses. The H1 components strongly differ also in their effects on enzymic DNA methylation: while 'typical' H1 has a strong inhibitory effect, its tightly-bound counterpart exerts a slight but definite stimulation.

A monoclonal antibody 5E5 recognizes an intranuclear antigen selectively present in a subpopulation of the neurons

Monoclonal antibody 5E5 labeled the nuclear antigen of the neurons in the guinea pig and rat central nervous systems including the cerebrum, cerebellum, spinal cord and retina. This antibody could discriminate neurons even among the same cell class. In in vitro study, only 10% of dividing PC12 cells was labeled with this antibody. An electron microscopic immunohistochemical study also revealed that this antibody selectively labeled heterochromatins in the neurons. Although we could not obtain any positive result by an immunoblot study, the antigenicity was remarkably diminished by the DNase I or S1 nuclease treatment on the tissue sections whereas RNase and trypsin was ineffective. These results suggested that this antigen might be a single-stranded DNA-protein complex resistant to proteolytic procedures, and possibly related to cell function or state of differentiation.

A possible role of chromatin and tightly-bound chromatin proteins on enzyme-catalyzed methylation of DNA

Upon extensive digestion with DNAaseI of placenta chromatin matrix, previously "stripped" from its loosely-bound components by high-salt extraction, a fraction is obtained that contains almost no endogenous DNA methylase activity but whose DNA, if still included in this whole fraction--not if it has been purified to a protein-free condition--is a good substrate for externally added enzyme. This chromatin matrix can even cause a significant stimulation of methylation of single-stranded Micrococcus luteus DNA by placental methylase. In vivo, this phenomenon may have possible counterparts in the existence of highly-methylated regions of chromatin loops that appear to be protected by tightly-bound protein components from digestion of the "stripped loops" with DNAaseI.

Poly(ADP-ribosylation) of nuclear proteins in rat testis correlates with active spermatogenesis

Poly(ADP-ribosylation) of nuclear proteins has been investigated in rat testis under different experimental conditions to determine whether it is associated with somatic or germinal cells. Isolated, intact nuclei were incubated with [14C]NAD and extracted sequentially with 5% HClO4 and 0.25 M HCl, and labelled soluble proteins were analysed by reverse-phase high-performance liquid chromatography and acetic acid-urea polyacrylamide gel electrophoresis (pH 2.9). Results show that in normal adult testis a major acceptor protein for poly(ADP-ribose) in HClO4 extracts is the tissue-specific histone, H1t. Core histones and three proteins (alpha, beta and gamma) with low mobility on acetic acid-urea gels were the major acceptors identified in HCl extracts. Poly(ADP-ribosylation) of all the aforementioned proteins is very low in isolated intact nuclei of testis from 8-day-old animals (only spermatogonia present in seminiferous tubules), increases significantly by 16-day (pachytene spermatocytes appear) and reaches adult proportions by 32 days (condensing spermatids present). In the nuclei from cryptorchid testes, poly(ADP-ribosylation) of nuclear proteins resembles 8-day-old testis. It is concluded that (a) poly(ADP-ribosylation) of nuclear proteins in rat testis is closely correlated with spermatogenesis and can be inferred that is particularly active in the early stages of meiosis; (b) testis-specific proteins (histone H1t and low mobility proteins, alpha, beta and gamma) are poly(ADP-ribosylated) to higher specific radioactivity than somatic histones.

Localization, in human placenta, of the tightly bound form of DNA methylase in the higher order of chromatin organization

In human placenta, the DNA of all subfractions of the third level of chromatin organization exhibits similar values of the methylcytosine-to-cytosine ratio. The tightly bound form of DNA methyltransferase is mostly recovered in the 'stripped loop' fraction, although, on the basis of the DNA content, the 'stripped loops' and the 'stripped matrix' appear to possess a similar amount of the enzyme. DNA methyltransferase activity is instead totally absent from the 'digested matrix', i.e., from the fraction remaining after digestion of the 'stripped matrix' with DNAase I. Upon addition of exogenous DNA methyltransferase, however, the DNA of this fraction, which is only 1% (in weight) of the total chromatin DNA and which has a length of approx. 9 kbp, can readily undergo methylation.

Tightly-bound non-histone proteins in different nucleosome-like subpopulations from pig kidney chromatin

By differential sucrose gradient centrifugation of pig kidney chromatin in the presence or absence of Na-EDTA and under varying ionic strength conditions, three nucleosome-like subpopulations with different buoyant densities can be obtained. These particles, on the basis of their histones and HMG protein pattern, of the 5-methylcytosine level of their DNA and of the RNA polymerase activity associated with them, can be considered as originating from chromatin fractions differently involved in gene expression. Two-dimensional electrophoresis of the tightly-bound non-histone proteins shows a distinct pattern for each subpopulation, such protein components being notably present in restricted numbers but in high amounts in the subpopulation which was apparently derived from condensed heterochromatin.

5-Methylcytosine levels in nucleosome subpopulations differently involved in gene expression

Sucrose density gradient centrifugation in the presence or absence of Na-EDTA and at different ionic strengths allows one to obtain well-defined nucleosome subpopulations the DNA of which, examined by gas chromatography-mass spectrometry, is in all cases hypermethylated as compared to spacer regions, but to a different extent for the different subpopulations. The various nucleosomes differ also in their content of histones and of high-mobility-group proteins, as well as in the levels of RNA polymerase activity associated with them. Such data suggest that these nucleosome subpopulations originate from chromatin fractions differently involved in gene expression.