Visualization of anti-histone antibodies on SV40 minichromosomes by scanning transmission electron microscopy (STEM)

Chromatin reconstitution on small DNA rings. III. Histone H5 dependence of DNA supercoiling in the nucleosome

Mononucleosomes were reconstituted on small DNA rings in the presence of histone H5 and relaxed to an equilibrium using calf thymus topoisomerase I. DNA products, when compared to the equilibria observed with the same minicircles in the absence of histones, showed that a linking number reduction of 1.6 to 1.7 was associated with this reconstitution, in contrast with the 1.1 to 1.2 figure reported in our recent study of the H5-free nucleosome. Gel electrophoretic properties and electron microscopic visualization of the nucleosomes suggest a correlation between this increase and a further wrapping of the DNA around the histone core from less than 1.5 turns of the superhelix in the absence of H5, to close to two turns in its presence. Implications for DNA topology in chromatin are discussed.

Characterization of SV40 chromatin by mass determination on STEM

Direct mass determination of purified SV40 minichromosomes was obtained by scanning transmission electron microscopy. Twenty to thirty percent of the minichromosomes were found with an Mr of 6.9 +/- 0.4 X 10(6). The rest of the molecules formed a spread Mr distribution ranging from 7.3 X 10(6) to 9.5 X 10(6) due possibly to different contents of the virus-coded proteins, mainly VP1. The apparent mass histogram of individual SV40 nucleosomes presents three maxima at Mr 2.1 X 10(5), 2.6 X 10(5) and 3.1 X 10(5) that could correspond to partially unravelled nucleosomes, complete nucleosomes and complete nucleosomes with the addition of VP1. Beaded structures with a higher mass were also measured; some were found at either side of the open nucleosome-free region.

T-antigen is the only detectable protein on the nucleosome-free origin region of isolated simian virus 40 minichromosomes

A nucleosome-free region or nucleosome gap, containing the origin of replication and the transcriptional promoter elements, is observed on 20%-25% of the SV40 minichromosomes isolated at physiological ionic strength at late time during the infectious cycle. We found that this subpopulation of gapped minichromosomes was more sensitive to digestion with a variety of single-cut restriction enzymes than the rest of the minichromosomes. This increased digestibility of gapped minichromosomes allowed us to excise the gap region by concomitant digestion with Bgl I and Msp I. T-antigen was the only detectable protein bound to this isolated chromatin fragment. In particular no histones could be detected. The presence of T-antigen on the gap region was confirmed by immunoelectron microscopy. Most of the T-antigen appeared to be located on the late side of the Bgl I restriction enzyme site.

Structural organization of an active, chromosomal nucleolar organizer region (NOR) identified by light microscopy, and subsequent TEM and STEM electron microscopy

The three-dimensional arrangement of the chromatin components within the nucleolar organizer regions (NORs) from living oocyte nuclei was investigated. As a suitable cell system we chose vitellogenic oocytes of the orthopteran insect Acheta. This cell type is particularly attractive for analysis of nucleolar chromatin, since structural and functional aspects of NORs during early oogenic stages (including the association of NORs with amplified rDNA copies) are particularly well known (Lima-de-Faria 1974). In the course of the present study we first identified putative chromosomal NORs in isolated nuclei of mid-diplotene oocytes according to morphological characteristics using differential interference contrast (DIC) or phase contrast light microscopy. The presence of actively transcribing chromosomal NORs during this late stage of Acheta oogenesis obviously had been overlooked by previous investigators, probably due to difficulties of chromosome visualization. For a more detailed ultrastructural analysis, NORs were gently sedimented from opened nuclei and processed for sectioning using a modified "end-embedding" procedure (Mott and Callan 1975; Spring and Franke 1981). A small number of thick and thin sections could be made from individual NORs. Sections were analyzed by light microscopy, conventional transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). Whereas the structural connection of NORs to the chromosome axis and also the general arrangement of active nucleolar genes within the NOR complex could be seen with TEM, the visualization of individual nucleolar genes and the organization of transcription complexes was only possible using bright field STEM of thick sections at low temperature.

Induction of altered chromatin structures by simian virus 40 enhancer and promoter elements

Two simian virus 40 transcriptional promoter elements, the 72-base pair (bp) repeat and the 21-bp repeat region, induce chromatin structures of increased DNase I sensitivity when transposed elsewhere in the viral genome. The induction of a sufficiently long stretch of DNase I-sensitive chromatin leads to the appearance of a visible nucleosome-free region.