SUMMARY Gene positioning and regulation of nuclear architecture are thought to influence gene expression. Here, we show that, in mouse olfactory neurons, silent olfactory receptor (OR) genes from different chromosomes converge in a small number of heterochromatic foci. These foci are OR exclusive and form in a cell-type-specific and differentiation-dependent manner. The aggregation of OR genes is developmentally synchronous with the downregulation of lamin b receptor (LBR) and can be reversed by ectopic expression of LBR in mature olfactory neurons. LBR-induced reorganization of nuclear architecture and disruption of OR aggregates perturbs the singularity of OR transcription and disrupts the targeting specificity of the olfactory neurons. Our observations propose spatial sequestering of heterochromatinized OR family members as a basis of monogenic and monoallelic gene expression.

Soft X-ray microscopes can be used to examine whole, hydrated cells up to 10 microm thick and produce images approaching 30 nm resolution. Since cells are imaged in the X-ray transmissive "water window", where organic material absorbs approximately an order of magnitude more strongly than water, chemical contrast enhancement agents are not required to view the distribution of cellular structures. Although living specimens cannot be examined, cells can be rapidly frozen at a precise moment in time and examined in a cryostage, revealing information that most closely approximates that in live cells. In this study, we used a transmission X-ray microscope at photon energies just below the oxygen edge (lambda = 2.4 nm) to examine rapidly frozen mouse 3T3 cells and obtained excellent cellular morphology at better than 50 nm lateral resolution. These specimens are extremely stable, enabling multiple exposures with virtually no detectable damage to cell structures. We also show that silver-enhanced, immunogold labelling can be used to localize both cytoplasmic and nuclear proteins in whole, hydrated mammary epithelial cells at better than 50 nm resolution. The future use of X-ray tomography, along with improved zone plate lenses, will enable collection of better resolution (approaching 30 nm), three-dimensional information on the distribution of proteins in cells.

We report the first experimental recording, to our knowledge, of the diffraction pattern from intact Escherichia coli bacteria using coherent x-rays with a wavelength of 2 Å. By using the oversampling phasing method, a real space image at a resolution of 30 nm was directly reconstructed from the diffraction pattern. An R factor used for characterizing the quality of the reconstruction was in the range of 5%, which demonstrated the reliability of the reconstruction process. The distribution of proteins inside the bacteria labeled with manganese oxide has been identified and this distribution confirmed by fluorescence microscopy images. Compared with lens-based microscopy, this diffraction-based imaging approach can examine thicker samples, such as whole cultured cells, in three dimensions with resolution limited only by radiation damage. Looking forward, the successful recording and reconstruction of diffraction patterns from biological samples reported here represent an important step toward the potential of imaging single biomolecules at near-atomic resolution by combining singleparticle diffraction with x-ray free electron lasers. Single-particle diffraction is a methodology of extending crystallography to determine the 2D and 3D structures of nano crystals and noncrystalline samples by using coherent x-rays and electrons (1-6). In this approach, coherent diffraction patterns are recorded and then converted directly to highresolution images by using the oversampling phasing method (7). Due to the lack of multiple copies of the object, which constructively reinforce due to Bragg diffraction, the diffraction patterns are usually weak and continuous. This illustrates why the experiments with high Z scatters, such as those made of Au and Ni, are simpler to carry out (2-4). However, some of the most important potential applications of single-particle diffraction lie in biological materials (8, 9). Here we report the first successful experiment, to our knowledge, of imaging Escherichia coli bacteria at 30-nm resolution by using single-particle x-ray diffraction. The images show the distribution of manganese-tagged proteins, which is consistent with results from fluorescence microscopy. MethodsExperimental Setup. The coherent diffraction experiment was conducted on an undulator beamline at SPring-8 (10). Because single-particle x-ray diffraction requires good spatial (i.e., beam divergence) and moderate temporal coherence (i.e., energy spread) (4), we achieved the beam divergence of Ϸ6 ϫ 10 Ϫ6 rad by setting a 150-m pinhole at a distance of 27 m upstream of the experimental instrument, and the energy spread of 0.7 eV (1 eV ϭ 1.602 ϫ 10 Ϫ19 J) by using a Si (1,1,1) double crystal (10). To make a small clean beam, a 20-m pinhole and a corner were placed at a distance of 25.4 and 12.7 mm upstream of the sample. The pinhole and corner combination created three very clean quadrants on a charge-coupled device (CCD) detector for recording weak diffraction patterns. The intensity lost in the noisy fourth quadrant was recover...

Mitochondrial fission is important for organelle transport, quality control and apoptosis. Changes to the fission process can result in a wide variety of neurological diseases. In mammals, mitochondrial fission is executed by the GTPase dynamin-related protein 1 (Drp1; encoded by DNM1L), which oligomerizes around mitochondria and constricts the organelle. The mitochondrial outer membrane proteins Mff, MiD49 (encoded by MIEF2) and MiD51 (encoded by MIEF1) are involved in mitochondrial fission by recruiting Drp1 from the cytosol to the organelle surface. In addition, endoplasmic reticulum (ER) tubules have been shown to wrap around and constrict mitochondria before a fission event. Up to now, the presence of MiD49 and MiD51 at ER-mitochondrial division foci has not been established. Here, we combine confocal live-cell imaging with correlative cryogenic fluorescence microscopy and soft x-ray tomography to link MiD49 and MiD51 to the involvement of the ER in mitochondrial fission. We gain further insight into this complex process and characterize the 3D structure of ER-mitochondria contact sites.

A theoretical analysis of the oscillating voltage at twice the Larmor frequency (2ω0), due to quadrupolar polarization of 14N nuclei, predicts a signal at least ten orders of magnitude smaller than the magnetic-induction signal produced by the same sample. However, nonsecular parts of the quadrupolar interaction are predicted to give rise to observable magnetic-induction signals at 2ω0 that provide a monitor of "two-quantum coherence." The theoretical prediction is verified using a single crystal of NaNO2 at room temperature. The corresponding signal from a crystalline powder is zero. Some limitations and possible extensions of the experiment are proposed.

Soft X-ray microcopy has resolved 30 nm structures in biological cells. To protect the cells from radiation damage caused by X-rays, imaging of the samples has to be performed at cryogenic temperatures, which makes it possible to take multiple images of a single cell. Due to the small numerical aperture of zone plates, X-ray objectives have a depth of focus on the order of several microns. By treating the X-ray microscopic images as projections of the sample absorption, computed tomography (CT) can be performed. Since cryogenic biological samples are resistant to radiation damage, it is possible to reconstruct frozen-hydrated cells imaged with a full-field X-ray microscope. This approach is used to obtain three-dimensional information about the location of specific proteins in cells. To localize proteins in cells, immunolabelling with strongly X-ray absorbing nanoparticles was performed. With the new tomography setup developed for the X-ray microscope XM-1 installed at the ALS, we have performed tomography of immunolabelled frozen-hydrated cells to detect protein distributions inside of cells. As a first example, the distribution of the nuclear protein, male specific lethal 1 (MSL-1) in the Drosophila melanogaster cell was studied.

No abstract

Cryo transmission x-ray microscopy in the "water window" of photon energies has recently been introduced as a method that exploits the natural contrast of biological samples. We have used cryo tomographic x-ray imaging of the intraerythrocytic malaria parasite, Plasmodium falciparum, to undertake a survey of the cellular features of this important human pathogen. We examined whole hydrated cells at different stages of growth and defined some of the structures with different x-ray density, including the parasite nucleus, cytoplasm, digestive vacuole and the hemoglobin degradation product, hemozoin. As the parasite develops from an early cup-shaped morphology to a more rounded shape, puncta of hemozoin are formed; these coalesce in the mature trophozoite into a central compartment. In some trophozoite stage parasites we observed invaginations of the parasite surface and, using a selective permeabilization process, showed that these remain connected to the RBC cytoplasm. Some of these invaginations have large openings consistent with phagocytic structures and we observed independent endocytic vesicles in the parasite cytoplasm which appear to play a role in hemoglobin uptake. In schizont stage parasites staggered mitosis was observed and x-ray-dense lipid-rich structures were evident at their apical ends of the developing daughter cells. Treatment of parasites with the antimalarial drug artemisinin appears to affect parasite development and their ability to produce the hemoglobin breakdown product, hemozoin. KeywordsPlasmodium; digestive vacuole; hemoglobin uptake; cryo x-ray tomography; soft x-ray microscopy