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The filamentous cyanobacteria Nostoc punctiforme ATCC 29133 is a model species fordevelopment of sustainable production methods of numerous compounds. One of its uniquefeatures is the anaerobic environment of the strains nitrogen fixing heterocyst cells. To be ableto properly utilize this environment, more knowledge regarding what regulates cell specificexpression is required. In this study, three motifs of the NsiR I promoter of Anabaena sp.PCC 7120 was studied in this system utilizing YFP-fluorescence as a reporter to determinetheir impact on spatial expression pattern. Investigations were performed on immobilizedcells with the use of confocal microscopy and results point towards sigma factor regulation.

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The three-dimensional (3-D) structure of gp32*I, a major proteolytic fragment of the DNA helix-destabilizing protein from bacteriophage T4, has been determined at 18 A resolution by electron microscopy of negatively stained crystals and computer image analysis. The crystalline areas processed in 3-D have the symmetry of the space group P2₁, with a = 47 Å, b = 63 Å, c = 65 Å, and α = β = γ = 90°. This P2₁ unit cell contains one gp32*I molecule per asymmetric unit. The molecule is roughly V-shaped, containing two large domains linked by a smaller domain occupying the base of the V. The total length of the molecule is about 110 Å with an average diameter of about 25 Å. Systematic analysis of the symmetry in images of untilted crystals determined that the crystal could display several types of projection symmetry, pgg, pg corresponding to P2₁ symmetry with the screw axis along the a axis of the crystal, and pg corresponding to P2₁ symmetry with the screw axis along the b axis. Among images displaying pg symmetry along the b axis, two types of images with noticeably different appearances were obtained. A hypothesis was formed that explained the different types of symmetry as the result of the growth of the gp32*I crystal in the space group P2₁ 2₁ 2₁, in steps of 1/2 of a unit cell along the thin direction of the crystal. Two different types of 1/2 unit cell thick steps were postulated. Computer simulations were used to generate synthetic images of untilted crystals containing either one, two or three steps of each kind. The results of the simulations prove that the space group of the gp32*I crystal is P2₁ 2₁ 2₁. They suggest that careful analysis of the symmetry in images of untilted gp32*I crystals can provide information about the thickness of the crystals. A strategy is presented for determining the structure of the gp32*I crystal at higher resolution by electron microscopy of frozen, hydrated crystals. This strategy includes the use of symmetry analysis as a tool for determining the thickness of the crystals so that data from crystals of the same thickness can be combined in 3-D. A similar approach may prove useful in the 3-D electron microscopic analysis of other thin, multi-layered crystals.

Using scanning probe microscopy, several studies were carried out to characterize surface topographies and properties. First, utilizing scanning tunneling microscopy (STM), we characterized fullerenes deposited onto gold foils and highly oriented gold films. On gold foils, we found that C₆₀ packed in hexagonally ordered overlayers and that the images showed internal buckyball features that arose from electronic interactions between the molecule and the substrate. On gold films, with an ordered overlayer of methyl isobutyl ketone (MIBK), the isolated C₆₀ molecules showed internal features in a "doughnut" shape, different than those seen previously. We also imaged gold foils on which a significant number of larger fullerene molecules were deposited, and found only spherical molecules in our images. A theoretical analysis of the optical beam deflection atomic force microscope (AFM) predicted sufficient sensitivity to measure atomic corrugations greater than 1 A. This agreed with experimental results showing atomically resolvable images. Another theoretical investigation probe the relative magnitude of the forces between the tip, sample, and an adsorbed atom on a surface. Experimentally, we investigated cleaved multiple quantum wells ans showed surface corrugations with a period equal to the quantum well spacing. The third technique used was magnetic force microscopy (MFM). We analyzed a novel system that combined the tunneling aspects of STM with the force-sensing attributes of force microscopy, and provided the ability to simultaneously image surface features as well as magnetic domains with a sensitivity that depended on the spring constant of the tunneling tip. Experimentally, we used this system to image magnetic domains and reveal the surface roughness of magnetic recording media. The second MFM technique involved spin-coating a magnetic surface with a ferrofliud, then over-coating with gold, and finally imaging the surface with STM. The STM revealed raised ridges where the ferromagnetic particles clumped in regions of high magnetic field gradient. The finally MFM we utilized imaged magnetic fields using a beam deflection force microscope by modulating a magnetic disk head and detecting the vibration of the magnetic tip. We were able to image the fields of a floppy disk head.

Semiconductors are characterized by atomic resolution imaging and density of states measurements (DOS) obtained through the use of a scanning tunneling microscope (STM). The DOS of the conduction and valence bands can be measured separately with a STM as opposed to an optical measurement which measures only the joint DOS. Layered-structure semiconductors are characterized both in the bulk form and in the isolated cluster form. Images of three bulk layered-structure semiconductors, MoS₂, WSe₂, and SnS₂, were obtained with both positive and negative sample-to-tip bias voltages. Curves of tunneling current as a function of bias voltage were measured, from which the DOS of the valence and conduction bands can be inferred. We obtained an atomically resolved image of an isolated fragment of a semi-conductor cluster which was deposited on a graphite surface from a colloidal suspension of BiI₃. Also imaged were clusters of MoS₂ layered-structure semiconductors.

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