Substantial Histone Reduction Modulates Genomewide Nucleosomal Occupancy and Global Transcriptional Output

The basic unit of genome packaging is the nucleosome, and nucleosomes have long been proposed to restrict DNA accessibility both to damage and to transcription. Nucleosome number in cells was considered fixed, but recently aging yeast and mammalian cells were shown to contain fewer nucleosomes. We show here that mammalian cells lacking High Mobility Group Box 1 protein (HMGB1) contain a reduced amount of core, linker, and variant histones, and a correspondingly reduced number of nucleosomes, possibly because HMGB1 facilitates nucleosome assembly. Yeast nhp6 mutants lacking Nhp6a and -b proteins, which are related to HMGB1, also have a reduced amount of histones and fewer nucleosomes. Nucleosome limitation in both mammalian and yeast cells increases the sensitivity of DNA to damage, increases transcription globally, and affects the relative expression of about 10% of genes. In yeast nhp6 cells the loss of more than one nucleosome in four does not affect the location of nucleosomes and their spacing, but nucleosomal occupancy. The decrease in nucleosomal occupancy is non-uniform and can be modelled assuming that different nucleosomal sites compete for available histones. Sites with a high propensity to occupation are almost always packaged into nucleosomes both in wild type and nucleosome-depleted cells; nucleosomes on sites with low propensity to occupation are disproportionately lost in nucleosome-depleted cells. We suggest that variation in nucleosome number, by affecting nucleosomal occupancy both genomewide and gene-specifically, constitutes a novel layer of epigenetic regulation

Effect of cryo-preservation on the response of different biological systems to gamma-ray exposure: A feasibility study

Cryopreservation of cells, tissues or organisms (i.e. the storage at liquid-nitrogen or nitrogen vapour temperature) offers the most secure form of preservation, allowing cells to be maintained unaltered for a long time and preventing them from ageing. On the other hand, the multi-year exposure of cryo-preserved cells (like: stem and germ cells) to the environmental γ-ray background might induce a radiation-damage accumulation, due to the inhibition of cellular repair mechanisms, and contribute to cancer or non-cancer pathology risk assessment when such cells should be transplanted in individuals. To investigate the effect of the cryo-preservation on the cell response to radiation exposures, the response of different biological systems (bacteria and mammalian cells) to γ-rays has been evaluated after irradiation in cryo-preserved and culture conditions, as a function of dose and doserate, in terms of a variety of cellular and molecular end-points