Histone H2A/H2B dimer exchange by ATP-dependent chromatin remodeling activities

ATP-dependent chromatin remodeling activities function to manipulate chromatin structure during gene regulation. One of the ways in which they do this is by altering the positions of nucleosomes along DNA. Here we provide support for the ability of these complexes to move nucleosomes into positions in which DNA is unraveled from one edge. This is expected to result in the loss of histone-DNA contacts that are important for retention of one H2A/H2B dimer within the nucleosome. Consistent with this we find that several chromatin remodeling complexes are capable of catalyzing the exchange of H2A/H2B dimers between chromatin fragments in an ATP-dependent reaction. This provides eukaryotes with additional means by which they may manipulate chromatin structure.</p

Nucleosome dynamics

In the 30 years since the discovery of the nucleosome, our picture of it has come into sharp focus. The recent high-resolution structures have provided a wealth of insight into the function of the nucleosome, but they are inherently static. Our current knowledge of how nucleosomes can be reconfigured dynamically is at a much earlier stage. Here, recent advances in the understanding of chromatin structure and dynamics are highlighted. The ways in which different modes of nucleosome reconfiguration are likely to influence each other are discussed, and some of the factors likely to regulate the dynamic properties of nucleosomes are considered.</p

Nucleosome dynamics

In the 30 years since the discovery of the nucleosome, our picture of it has come into sharp focus. The recent high-resolution structures have provided a wealth of insight into the function of the nucleosome, but they are inherently static. Our current knowledge of how nucleosomes can be reconfigured dynamically is at a much earlier stage. Here, recent advances in the understanding of chromatin structure and dynamics are highlighted. The ways in which different modes of nucleosome reconfiguration are likely to influence each other are discussed, and some of the factors likely to regulate the dynamic properties of nucleosomes are considered.</p

Nucleosomes can invade DNA territories occupied by their neighbors

Nucleosomes are the fundamental subunits of eukaryotic chromatin. They are not static entities, but can undergo a number of dynamic transitions including spontaneous repositioning along DNA. Since nucleosomes are spaced close together within genomes it is likely that on occasion they approach each other and or collide. Here we have used a dinucleosomal model system to show that the 147bp DNA territories of two nucleosomes can overlap extensively. In the situation of an overlap by 44 bp or 54 bp one histone dimer is lost and the resulting complex can condense to form a compact single particle. We propose a pathway in which adjacent nucleosomes promote DNA unraveling as they approach each other and that this permits their 147bp territories to overlap. These may represent early steps in a pathway for nucleosome removal via collision. In eukaryotic cells genomic DNA exists in the form of a nucleo-protein complex called chromatin 1. The packaging of the genomic DNA imposes a hindrance to most DNA-dependent processes including DNA replication, repair and mRNA transcription. This implies an important role for chromatin structure in the control of many nuclear functions 2,3. The first step in the packaging hierarchy of chromatin is the formation of a nucleosom