Thomas F. Anderson

The infection of Escherichia coli by T2 and T4 bacteriophages as seen in the electron microscope II. Structure and function of the baseplate

Abstract The baseplate of the T-even bacteriophage seems to play a central role in the infection of the host. In the native particle it is attached both to the extended sheath and to the tip of the central hollow needle. It has the shape of a flat hexagon with a short pin at each corner and a trigonal plug in its center. In addition, six long tail fibers are attached to its vertices. These fibers are responsible for the initial attachment of the particle to the susceptible host cell. Subsequently, the short pins become attached to the cell surface. A series of structural changes in the baseplate and the sheath follow. The baseplate becomes a six-pointed star with the tips pointing diagonally away from the bacterial surface and with the six long tail fibers attached to them. The baseplates attachment to the needle is dissolved and an 80 A hole is found instead of the baseplates central plug. These transformations in the baseplate may initiate the contraction of the sheath. Since the sheath is connected to the needle near the neck of the particle, its contraction thus causes the needle to slide through the baseplate and press against the bacterial wall. This forces the baseplate away from the cell wall and creates tension in the pins which are drawn out into short tail fibers. It seems that the needle is thus forced to penetrate the cell wall (which may be weakened by phage enzymes) and to serve as a channel through which the phages DNA may pass into the cell.

Fractionation of Shope papilloma virus in cesium chloride density gradients

Abstract Virus particles prepared from extracts of glycerinated Shope papilloma tissue of wild cottontail rabbits by means of three alternate low and high speed sedimentations were not uniform in the electron microscope. Variations in size and electron density were seen in negative-contrast preparations. When such particles were centrifuged to equilibrium in a cesium chloride density gradient they separated into four layers corresponding to densities of about 1.29, 1.32, 1.33, and 1.34. Ultraviolet absorption studies, infectivity tests, and the homogeneity of the particles were consistent with the thesis that the most dense particles contained the highest proportion of deoxyribonucleic acid (DNA) and were probably the complete virus. The intermediate layers appeared to have less DNA, and the top layer little or none. In electron micrographs the most dense particles were uniform. Their surfaces were composed of an estimated 60 to 70 capsomeres. The particles in the intermediate layers showed various degrees of emptiness at their centers. The lightest particles appeared to be loose aggregates of capsomeres. When the particles were exposed to fresh cesium chloride, they appeared to become more fragile, many of them disintegrating into free capsomeres on the electron microscope grid. These subunits of the protein coat had a structure suggestive of hollow cylinders or cups. Subunits of the viral core could not be definitely characterized in density gradients or visualized by means of the electron microscope.

Genomic masking and recombination between serologically unrelated phages P22 and P221.

Abstract The bacteriophage P22 has a short tail with a hexagonal base plate but no contractile sheath. It is not adsorbed on a mutant strain ( St 22 ) of its host Salmonella typhimurium (St). A mutant P22h, forming faint plaques on St 22 and clear plaques on St, is indistinguishable morphologically from P22. Preparations of P22 also contain a small proportion of a long-tailed phage, P221, which forms plaques on St 22 but not on St, by which it is not adsorbed. Different strains of P22, containing markers like c+, c1 and c2 that affect lysogenization, produce P221 strains with corresponding markers. Neither P22 nor P22h cross reacts serologically with P221. However, mixed infection of St 22 by P22h and P221 produces masked genomes: particles carrying P22h genomes in P221 capsids and P221 genomes in P22h capsids. Moreover, P22h markers can be transferred to P221 genomes and vice versa. The existence of these stable hybrids may indicate that P221 represents a morphologically and serologically distinct mutant of P22. Alternatively, there may be present in St a defective prophage whose defects can be rectified by recombination with P22 to yield P221.

The reactions of bacterial viruses with their host cells

In its early stages the study of a sector of nature may be divided into two branches. In one we inquire what an object under investigation is; we want to know its morphology and its relation to other more or less similar objects. In the other branch we study what the object does and how it came to be what it is. Thus in chemistry we have molecular structure on the one hand and kinetics of reaction on the other. In biology, anatomy and embryology may be taken as examples of the two points of view. As a science progresses certain relations between structure and function begin to emerge, and eventually the pursuit of these relations may become the major interest in the field. The study of viruses is a very young science indeed. In recent years much progress has been made toward a better understanding of those phases of the virus problem concerned with the infectious agent or virus particle (7). Improved methods of isolation, culture, concentration and assay have made purified preparations of many viruses available in relatively large quantities for physical and chemical characterization. Application of the ultracentrifuge and diffusion techniques gives indirect evidence of the mean sizes and shapes of virus particles in solution; the electron microscope has made possible direct observation of sizes and shapes of individual particles in the dry state (7, 8, 53, 56, 68, 69, 72) ; and x-ray diffraction has given clues to the atomic architecture of certain plant virus particles (11). It has turned out that this segment of biological material includes particles as small as 10 mu and as large as 200 mu in diameter. The morphologies of the viruses range from relatively large cell-like structures (vaccinia), some of them being tadpole-shaped (T2 bacteriophage), to simple rods (tobacco mosaic) and spheres (bushy stunt). On the other hand, the particles of any particular strain of virus exhibit a remarkable uniformity in size and structure.

Electron Microscopic Studies of Biological Reactions. I. Reduction of Potassium Tellurite by Corynebacterium diphtheriae.

Summary (1) Typical polar granules appear as dense spherical masses, or possibly plates, in electron micrographs of unstained preparations of C. diphtheriae grown on blood infusion agar. (2) In addition to polar granules, needle-like crystals appear in electron micrographs of unstained preparations of C. diphtheriae cells grown on potassium tellurite chocolate agar. (3) The needle-like crystals, as well as the black color, of cell masses of C. diphtheriae grown on potassium tellurite chocolate agar disappear upon the addition of small amounts of bromine water. It is inferred, therefore, that the black color is due to the tellurium metal which occurs in the form of needles. (4) It is to be further inferred that the tellurite ion is able to diffuse through the cell wall and is reduced with the precipitation of tellurium metal within the cell boundaries. (5) With the aid of the electron microscope it is now possible to obtain pictorial records of the location of sites of certain chemical reactions incident to the metabolism of the bacterial cell.