Solution structure(s) of trinucleosomes from contrast variation SAXS

Nucleosomes in all eukaryotic cells are organized into higher order structures that facilitate genome compaction. Visualizing these organized structures is an important step in understanding how genomic DNA is efficiently stored yet remains accessible to information-processing machinery. Arrays of linked nucleosomes serve as useful models for understanding how the properties of both DNA and protein partners affect their arrangement. A number of important questions are also associated with understanding how the spacings between nucleosomes are affected by the histone proteins, chromatin remodelers, or other chromatin-associated protein partners. Contrast variation small angle X-ray scattering (CVSAXS) reports the DNA conformation within protein-DNA complexes and here is applied to measure the conformation(s) of trinucleosomes in solution, with specific sensitivity to the distance between and relative orientation of linked nucleosomes. These data are interpreted in conjunction with DNA models that account for its sequence dependent mechanical properties, and Monte-Carlo techniques that generate realistic structures for comparison with measured scattering profiles. In solution, trinucleosomes segregate into two dominant populations, with the flanking nucleosomes stacked or nearly equilaterally separated, e.g. with roughly equal distance between all pairs of nucleosomes. These populations are consistent with previously observed magnesium-dependent structures of trinucleosomes with shorter linkers.     

Inhibitors of the p53-Mdm2 interaction increase programmed cell death and produce abnormal phenotypes in the placozoon Trichoplax adhaerens (F.E. Schulze)

Recent identification of genes homologous to human p53 and Mdm2 in the basal phylum Placozoa raised the question whether the network undertakes the same functions in the most primitive metazoan organism as it does in more derived animals. Here, we describe inhibition experiments on p53/Mdm2 interaction in Trichoplax adhaerens by applying the inhibitors nutlin-3 and roscovitine. Both inhibitors had a strong impact on the animals’ survival by significantly increasing programmed cell death (cf. apoptosis, measured via terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling assay). Treatment with roscovitine decreased cell proliferation (visualized by means of bromodeoxyuridine incorporation), which is likely reducible to its function as cyclin-dependent kinase inhibitor. Obvious phenotypic abnormalities have been observed during long-term application of both inhibitors, and either treatment is highly lethal in T. adhaerens. The findings of this study suggest a conserved role of the p53/Mdm2 network for programmed cell death since the origin of the Metazoa and advocate the deployment of Placozoa as a model for p53, apoptosis, and possibly cancer research.