Filamentous Bacterial viruses

The filamentous bacterial virus is a simple and well-characterized model system for studying how genetic information is transformed into molecular machines. The viral DNA is a single-stranded circle coding for about 10 proteins. The major viral coat protein is largely α-helical, with about 46 amino acid residues. Several thousand identical copies of this protein in a helical array form a hollow cylindrical tube 1–2μ long, of outer diameter 60 Å and inner diameter 20 Å, with the twisted circular DNA extending down the core of the tube. Before assembly, the viral coat protein spans the cell membrane, and assembly involves extrusion of the coat from the membrane. X-ray fibre diffraction patterns of the Pf 1 species of virus at 4°C, oriented in a strong magnetic field, give three-dimensional data to 4 Å resolution. An electron density map calculated from native virus and a single iodine derivative, using the maximum entropy technique, shows a helix pitch of 5.9 Å. This may indicate a stretched A-helix, or it may indicate a partially 3₁₀ helix conformation, resulting from the fact that the coat protein is an integral membrane protein before assembly, and is still in the hydrophobic environment of other coat proteins after assembly.     

Letter: Filamentous bacterial viruses. X. X-ray diffraction studies of the R4-protein mutant

Eukaryotic DNA fragments that are totally denatured by alkali swiftly re-form duplex regions that are several hundred up to several thousand nucleotide pairs in length. A combination of sedimentation and electron microscopic studies demonstrate that they arise by the folding-back of a single linear chain, and not from cross-linking between the two complementary chains. Thus these “hairpin”-like structures must come from inverted repetitions of the type A B C t C′B′A′ that are located at intervals along the chromatid. Electron microscopic studies, reveal no unpaired single-chain regions in the “turn-around” t. The resistance of these hairpins to single-chain specific nucleases indicates that t must only consist of a few nucleotides. Therefore we call these regions in double-chain DNA palindromes, because, given the antiparallel arrangement of the polynucleotide chains, these sequences read the same both backwards and forwards. The thermal stability profile of these hairpins is nearly identical to that of sonicated duplex fragments of comparable length. Since these hairpins have the same average base composition as bulk DNA, the palindromes are nearly perfect. By studying the fraction of DNA retained on hydroxyapatite as a function of chain length, one may determine the distribution of palindromes along the DNA. These experiments are best explained by clusters of palindromes located at intervals of 10 to 80 /gm depending on the species. The presence of such long, well-matched palindromes suggests that the linear double helix may sometimes adopt an alternative configuration, the cruciform, in which mismatches that may occur are eliminated by excision and repair.     

Filamentous bacterial viruses. XI. Molecular architecture of the class II (Pf1, Xf) virion

The diffraction patterns of the Pf 1 and Xf strains of filamentous bacterial viruses (class II) can be interpreted in terms of a simple helix of protein subunits with 15Åpitch, having 22 units in five turns. The protein subunits are each elongated in an axial direction, and also slope radially, so as to overlap each other, giving an arrangement of subunits reminiscent of scales on a fish. The protein helix forms a tube with inner diameter about 20Åand outer diameter about 60Å. The single-stranded circular DNA is contained within this tube, with two DNA strands running the length of the tube.The diffraction patterns of fd, If 1 and IKe (class I) can be interpreted in terms of a perturbed version of the class II simple helix.     

Filamentous bacterial viruses. XIL. Molecular architecture of the class I (fd, If1, IKe) virion

The X-ray diffraction patterns of the fd, If1 and IKe strains of filamentous bacterial viruses (class I) indicate that the arrangement of capsid proteins in the virion approximates a left-handed helix of 15 Å pitch with 4.5 units per turn. The protein molecules are each elongated in an axial direction, and also slope radially, so as to overlap each other and give an arrangement of molecules reminiscent of scales on a fish. This helix of capsid proteins is related to the class II helix by a small twist about the helix axis. The protein molecules are also perturbed (by a few Ångström units) away from the positions that they would occupy in a simple 4.5 units per turn helix. The perturbation repeats about every five protein molecules, and is mainly axial. This arrangement of proteins forms a tube with inner diameter about 20 Å and outer diameter about 60 Å, encapsulating the DNA.     

Action of Spirulina platensis on bacterial viruses

The impact of the biomass of the blue-green microalga (cyanobacterium) S. platensis on bacteriophage T4 (bacterial virus) has been evaluated. The study revealed that the addition of S. platensis biomass into the agar nutrient medium, followed by sterilization with 2% chloroform and thermal treatment, produced an inhibiting or stimulating effect on the reproduction of the bacteriophage in Escherichia coli B cells, depending on the concentration of S. platensis and the multiplicity of phage infection, as well as on the fact whether the microalgae were added during the first cycle of the development of the virus. The reproduction of the bacteriophage in E. coli B was influenced by the method and duration of the sterilization of the nutrient medium with S. platensis.     

Filamentous bacterial viruses : I. DNA synthesis during the early stages of infection with fd

During the first ten minutes after infection of bacteria with fd, the rate of DNA synthesis in an infected culture becomes several-fold larger than the rate in a parallel uninfected culture. This stimulation of rate is due to the synthesis of 100 to 200 double-stranded forms of viral DNA, superimposed on continuing bacterial DNA synthesis. At the end of the ten-minute period, the rate of viral plus bacterial DNA synthesis stops increasing, and remains constant for the next 50 minutes. The abrupt decrease in acceleration of net DNA synthesis corresponds in time to the onset of synthesis of single-stranded viral DNA.     

Surface inactivation of bacterial viruses and of proteins

1. The seven bacterial viruses of the T group active against E. coli, are rapidly inactivated at gas-liquid interfaces. 2. The kinetics of this inactivation whether brought about by shaking or by bubbling with nitrogen are those of a first order reaction. 3. This inactivation may be prevented by the addition of enough protein to maintain the gas-liquid interface in a saturated condition. 4. The analogy between this phenomenon and the surface denaturation of proteins is pointed out and discussed.