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The High Mobility Group Box (HMGB) chromosomal proteins have been known and studied for a long time, but we have only recently started to understand their biological functions. They now have a clear reputation for being important architectural factors: they facilitate the assembly of site‐specific DNA binding proteins to their cognate binding sites within chromatin. Beyond this intranuclear function, they also have an extracellular function, which will be the prime focus of this short review. ### The HMGB family: structure, expression and nuclear function The HMGB family comprises the three proteins HMGB1 (previously HMG1), HMGB2 (previously HMG2) and HMGB3 (previously HMG4 or HMG2b) (Bustin, 2001). The structure of these three proteins is highly conserved (>80% amino acid identity), and their biochemical properties are so far indistinguishable. HMGBs are composed of three different domains. The two homologous DNA binding domains, HMG boxes A and B, are each ∼75 amino acids in length. The C‐terminal domain is highly negatively charged, consisting of a continuous stretch of glutamate or aspartate residues, and is longest in HMGB1 and shortest in HMGB3 (reviewed in Bustin, 1999; Bianchi and Beltrame, 2000). HMGB1 is ubiquitous and only 10 times less abundant than core histones, at ∼106 molecules per typical mammalian cell. Expression of the other two family members is more restricted: HMGB3 is only expressed to a significant amount during embryogenesis (Vaccari et al ., 1998); HMGB2 is widely expressed during embryonic development, but restricted mainly to lymphoid organs and testis in the adult mouse (Ronfani et al ., 2001). The localization of these proteins in most cells is nuclear. In their nuclear identity, HMGB1 and HMGB2 bind to the minor groove of DNA, causing a local distortion of the double helix. They have little or no sequence preference, and they are recruited to the site of action by specific DNA binding proteins. HMGB1 has …

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. The accurate preservation and correct retrieval of genetic information is crucial for all living organisms. In eukaryotes, whether single-celled yeast or complex mammals, the DNA containing the genetic information is wrapped around beads of histone proteins to form structures called nucleosomes along the length of the DNA; this packaging arrangement helps protect the genome from damage and may restrict access to the genetic information. Until recently, the amount of histones and, consequently, the number of nucleosomes in the cell were considered fixed. Here, we show that in both mammalian and yeast cells that lack a single protein—HMGB1 in mammals or Nhp6a/b in yeast—the abundance of histones and nucleosomes decreases by 20%–30%. Contrary to expectations, we found that in yeast the nucleosomes do not redistribute along DNA when they are fewer: they largely maintain their positions, but the amount of time each specific DNA site spends wrapped in a nucleosome (i.e., its occupancy) decreases. Sequences that are already less frequently occupied than others in normal yeast cells lose disproportionally more nucleosomes in the mutant yeast that lack Nhp6a/b. This gives rise to a global increase in transcription and specific alterations in the expression of certain genes. This study thus contributes to a deeper understanding of how the DNA is packaged and organized. It also suggests that the cell's histone content might contribute in an important way to gene regulation.