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.

Haspin is an atypical protein kinase that in several organisms phosphorylates histone H3Thr3 and is involved in chromosome segregation. In Saccharomyces cerevisiae, H3Thr3 phosphorylation has never been observed and the function of haspin is unknown. We show that deletion of ALK1 and ALK2 haspin paralogs causes the mislocalization of polarisome components. Following a transient mitotic arrest, this leads to an overly polarized actin distribution in the bud where the mitotic spindle is pulled. Here it elongates, generating anucleated mothers and binucleated daughters. Reducing the intensity of the bud-directed pulling forces partially restores proper cell division. We propose that haspin controls the localization of polarity cues to preserve the coordination between polarization and the cell cycle and to tolerate transient mitotic arrests. The evolutionary conservation of haspin and of the polarization mechanisms suggests that this function of haspin is likely shared with other eukaryotes, in which haspin may regulate asymmetric cell division.