Biomedical subjects

Martin F Hohmann-Marriott

Publications and source records attributed to Martin F Hohmann-Marriott.

6 recordsLinked to original sources

PubMed2006

Variable fluorescence in green sulfur bacteria.

Green sulfur bacteria possess a complex photosynthetic machinery. The dominant light harvesting systems are chlorosomes, which consist of bacteriochlorophyll c, d or e oligomers with small amounts of protein. The chlorosomes are energetically coupled to the membrane-embedded iron sulfur-type reaction center via a bacteriochlorophyll a-containing baseplate protein and the Fenna-Matthews-Olson (FMO) antenna protein. The fluorescence yield and spectral properties of these photosynthetic complexes were investigated in intact cells of several species of green sulfur bacteria under physiological, anaerobic conditions. Surprisingly, green sulfur bacteria show a complex modulation of fluorescence yield upon illumination that is very similar to that observed in oxygenic phototrophs. Within a few seconds of illumination, the fluorescence reaches a maximum, which decreases within a minute of illumination to a lower steady state. Fluorescence spectroscopy reveals that the fluorescence yield during both processes is primarily modulated on the FMO-protein level, while the emission from chlorosomes remains mostly unchanged. The two most likely candidates that modulate bacteriochlorophyll fluorescence are (1) direct excitation quenching at the FMO-protein level and (2) indirect modulation of FMO-protein fluorescence by the reduction state of electron carriers that are part of the reaction center.

Bacterial Proteins↗

PubMed2006

Application of electron tomography to fungal ultrastructure studies.

Access to structural information at the nanoscale enables fundamental insights into many complex biological systems. The development of the transmission electron microscope (TEM) has vastly increased our understanding of multiple biological systems. However, when attempting to visualize and understand the organizational and functional complexities that are typical of cells and tissues, the standard 2-D analyses that TEM affords often fall short. In recent years, high-resolution electron tomography methods, coupled with advances in specimen preparation and instrumentation and computational speed, have resulted in a revolution in the biological sciences. Electron tomography is analogous to medical computerized axial tomography (CAT-scan imaging) except at a far finer scale. It utilizes the TEM to assemble multiple projections of an object which are then combined for 3-D analyses. For biological specimens, tomography enables the highest 3-D resolution (5 nm spatial resolution) of internal structures in relatively thick slices of material (0.2-0.4 microm) without requiring the collection and alignment of large numbers of thin serial sections. Thus accurate and revealing 3-D reconstructions of complex cytoplasmic entities and architecture can be obtained. Electron tomography is now being applied to a variety of biological questions with great success. This review gives a brief introduction into cryopreservation and electron tomography relative to aspects of cytoplasmic organization in the hyphal tip of Aspergillus nidulans.

Aspergillus nidulans↗

PubMed2005

The three-dimensional structure of the cyanobacterium Synechocystis sp. PCC 6803.

To advance our knowledge of the model cyanobacterium Synechocystis sp. PCC 6803 we investigated the three-dimensional organization of the cytoplasm using standard transmission electron microscopy and electron tomography. Electron tomography allows a resolution of ~5 nm in all three dimensions, superior to the resolution of most traditional electron microscopy, which is often limited in part by the thickness of the section (70 nm). The thylakoid membrane pairs formed layered sheets that followed the periphery of the cell and converged at various sites near the cytoplasmic membrane. At some of these sites, the margins of thylakoid membranes associated closely along the external surface of rod-like structures termed thylakoid centers, which sometimes traversed nearly the entire periphery of the cell. The thylakoid membranes surrounded the central cytoplasm that contained inclusions such as ribosomes and carboxysomes. Lipid bodies were dispersed throughout the peripheral cytoplasm and often juxtaposed with cytoplasmic and thylakoid membranes suggesting involvement in thylakoid maintenance or biogenesis. Ribosomes were numerous and mainly located throughout the central cytoplasm with some associated with thylakoid and cytoplasmic membranes. Some ribosomes were attached along internal unit-membrane-like sheets located in the central cytoplasm and appeared to be continuous with existing thylakoid membranes. These results present a detailed analysis of the structure of Synechocystis sp. PCC 6803 using high-resolution bioimaging techniques and will allow future evaluation and comparison with gene-deletion mutants.

Cytoplasm↗

PubMed2005

Digital position determination system for electron microscopy.

The precise determination of object positions within a specimen grid is important for many applications in electron microscopy. For example, real-time position determination is necessary for current statistical approaches and the efficient mapping and relocation of objects. Unfortunately, precise real-time position determination is not available on many older electron microscopes with manual stage controls. This report demonstrates the cost-effective and flexible implementation of a digital position determination system that can be adapted to many hand-operated electron microscopes. A customized solution that includes the hardware and software to accomplish position determination is presented. Lists of required parts, instructions for building the hardware, and descriptions of the developed programs are included. Two LED-photodiode assemblies detect x and y movements via an optical wheel that is in physical contact with the mechanical x and y stage control elements. These detector assemblies are interfaced with an integrated circuit that converts movement information into serial port-compatible signals, which are interpreted by a computer with specialized software. Two electron microscopes, a Philips CM12 (S)TEM and a Philips 201 TEM, were equipped with the described digital position determination system. The position fidelity and position fidelity after reloading of grids were determined for both microscopes. The determined position deviation was 1.06 microm in the x axis and 0.565 microm in the y axis for the Philips CM12 (S)TEM, and 0.303 microm in the x axis and 0.545 microm in the y axis for the Philips 201 TEM. After reloading and computational realigning, the determined average position variation was 2.66 microm in the x axis and 2.61 microm in the y axis for the Philips CM12 (S)TEM, and 1.13 microm in the x axis and 1.27 microm in the y axis for the Philips 201 TEM.

Electronics↗

PubMed2005

The ultrastructure of Chlorobium tepidum chlorosomes revealed by electron microscopy.

Chlorosomes are the light harvesting structures of green photosynthetic bacteria. Each chlorosome from green sulfur bacteria houses hundreds of thousands of bacteriochlorophyll molecules in addition to smaller amounts of chlorobiumquinone and carotenoids. In electron microscopy studies, chlorosomes exhibit different appearances depending on the fixation method used. Fixation with osmium tetroxide results in electron-transparent chlorosomes. Fixation with potassium permanganate results in clearly delineated electron-dense chlorosomes. This fixation method features an electron-transparent area in the interior of the chlorosome. In addition to electron density patterns that can be considered compositions of rod-shaped elements, chlorosomes exhibit a striation pattern that is oriented parallel to the longitudinal axis. Treatment with osmium tetroxide followed by potassium permanganate treatment results in a more diffused density distribution that outlines connecting elements between the chlorosome and the cytoplasmic membrane, and connecting elements between the cytoplasmic membrane and the outer membrane, which act as a diffusion barrier for electron density.

Cell Membrane↗

PubMed2005

Effect of iron on growth and ultrastructure of Acaryochloris marina.

The cyanobacterial genus Acaryochloris is the only known group of oxygenic phototrophs that contain chlorophyll d rather than chlorophyll a as the major photosynthetic pigment. Studies on this organism are still in their earliest stages, and biochemical analysis has rapidly outpaced growth optimization. We have investigated culture growth of the major strains of Acaryochloris marina (MBIC11017 and MBIC10697) by using several published and some newly developed growth media. It was determined that heavy addition of iron significantly enhanced culture longevity. These high-iron cultures showed an ultrastructure with thylakoid stacks that resemble traditional cyanobacteria (unlike previous studies). These cultures also show a novel reversal in the pigment ratios of the photosystem II signature components chlorophyll a and pheophytin a, as opposed to those in previous studies.

Chromatography, High Pressure Liquid↗