Steroid receptors bind to site-specific response elements in chromatin and modulate gene expression in a hormone-dependent fashion. With the use of a tandem array of mouse mammary tumor virus reporter elements and a form of glucocorticoid receptor labeled with green fluorescent protein, targeting of the receptor to response elements in live mouse cells was observed. Photobleaching experiments provide direct evidence that the hormone-occupied receptor undergoes rapid exchange between chromatin and the nucleoplasmic compartment. Thus, the interaction of regulatory proteins with target sites in chromatin is a more dynamic process than previously believed.

The repair of DNA double-strand breaks (DSBs) is facilitated by the phosphorylation of H2AX, which organizes DNA damage signaling and chromatin remodeling complexes in the vicinity of the lesion (Pilch, D.R., O.A. Sedelnikova, C. Redon, A. Celeste, A. Nussenzweig, and W.M. Bonner. 2003. Biochem. Cell Biol. 81:123–129; Morrison, A.J., and X. Shen. 2005. Cell Cycle. 4:568–571; van Attikum, H., and S.M. Gasser. 2005. Nat. Rev. Mol. Cell. Biol. 6:757–765). The disruption of DNA integrity induces an alteration of chromatin architecture that has been proposed to activate the DNA damage transducing kinase ataxia telangiectasia mutated (ATM; Bakkenist, C.J., and M.B. Kastan. 2003. Nature. 421:499–506). However, little is known about the physical properties of damaged chromatin. In this study, we use a photoactivatable version of GFP-tagged histone H2B to examine the mobility and structure of chromatin containing DSBs in living cells. We find that chromatin containing DSBs exhibits limited mobility but undergoes an energy-dependent local expansion immediately after DNA damage. The localized expansion observed in real time corresponds to a 30–40% reduction in the density of chromatin fibers in the vicinity of DSBs, as measured by energy-filtering transmission electron microscopy. The observed opening of chromatin occurs independently of H2AX and ATM. We propose that localized adenosine triphosphate–dependent decondensation of chromatin at DSBs establishes an accessible subnuclear environment that facilitates DNA damage signaling and repair.

We developed an X-ray microscope using partially coherent object illumination instead of previously used quasi-incoherent illumination. The design permitted the incorporation of a cryogenic tilt stage, enabling tomography of frozen-hydrated, intact adherent cells. We obtained three-dimensional reconstructions of mouse adenocarcinoma cells at ∼36-nm (Rayleigh) and ∼70-nm (Fourier ring correlation) resolution, which allowed us to visualize the double nuclear membrane, nuclear pores, nuclear membrane channels, mitochondrial cristae and lysosomal inclusions.