Computational modeling KU-60019 research buy of the chromatin fiber suggests that the nucleoprotein polymer is theoretically far less efficient for packaging than was previously assumed [8 and 9], and a series of experimental studies provide support for these computational models. Using cryo-electron microscopy (EM) coupled with careful measurements, 30 nm fibers were not detected in interphase nuclei, or even in metaphase chromosomes

[10• and 11•]. Using small-angle X ray scattering (SAXS), another group likewise reported it was unable to detect 30 nm fibers in vivo, but rather raised the startling possibility that the data which first reported 30 nm fibers might instead have been periodic reiterations of ribosomes, which are 30 nm in width and were found to coat the chromatin under certain preparative procedures [ 12•]. Despite the ongoing

debate on this issue [ 13], it does appear that much of the chromatin fiber exists in the 10 nm fiber state (beads on a string) ( Figure 1g), with a few locally folded areas comprising 5-10 nucleosomes and with 3D “fractal globule” arrangements of chromatin fibers stabilized in a cross-array format ( Figure 1h), the density of which is possibly coordinated by linker histone H1 and networks of non-histone proteins [ 14]. These results, along with the evolutionary evidence that Z-VAD-FMK supplier archaeal histones do not function as a packaging molecule, lend themselves to the possibility that histones may have evolved primarily as a means of regulating local access to genes [ 15 and 16]. Thus, if canonical histones generally serve to regulate access to the DNA, what additional roles do specialized histone variants Metalloexopeptidase play in regulating the various cellular processes that occur throughout the genome? All eukaryotes studied thus far contain the histone variant H3.3 and the centromere-specific histone variant CENP-A/CENH3, even when they lack other H3

types [17]. Additional variants include H2A.Z/HTZ, H2A.X, H2Av, H2A.Bbd, macroH2A [17], the primate-specific histones H3.X and H3.Y [18] (Table 1), and a plethora of histone H1 variants. Remarkably, while these proteins were discovered decades ago, their precise function, the mechanisms by which they effect change on the chromatin fiber, how they are inherited in vivo, and their contributions to the progression of disease states remain open questions in biology. In this review, we highlight recent advances and yet to be answered fundamental questions regarding the behavior of histone variants and their influence on cellular function in the normal and diseased states. The histone variants H3.3 and H2A.Z have both been individually linked to a role in regulating transcription, but biochemical purification suggests that these two variants may come together in a single nucleosome. Using HeLa cells expressing a Flag-tagged H3.