Structural Properties and Features of Parasitic Bdellovibrio bacteriovorus

The structure of five parasitic strains of Bdellovibrio bacteriovorus was studied by electron microscope after negative staining and in shadow-case and etched freeze-fractured preparations. Special attention was paid to the cell wall and the flagellar sheath which is continuous with the wall or part of it. These structural components reveal distinct features which are induced by certain staining substances; they are exceedingly susceptible to disruption by physical treatments, and in old cells often appear impaired. In freeze-fractured cells the wall shows characteristic fracturing tendencies not known in other microorganisms. These structural properties and features are distinct to Bdellovibrio wall and flagellar sheath, the structural integrity of which is a fundamental requirement for the infectivity and survival of this organism. The anterior end of Bdellovibrio is differentiated: 6 to 12 ring-like structures (9 to 12 nm, outer diameter) are built into its wall and several fibers (7 to 10 nm wide, up to 1.5 μm long) emerge from it. Intracellular structures, which are revealed as compact oval bodies bulging from the cell border and have internal laminated organization, are characteristic of Bdellovibrio after negative staining with certain compounds. These findings on the structure of parasitic Bdellovibrio substantiate previous observations indicating the uniqueness of this organism and add criteria for the identification of this genus.     

Relationships between liquid- and solid-phase antibody association characteristics: implications for the use of competitive ELISA techniques to map the spatial location of idiotopes

A liquid-phase assay system based on small-zone size-exclusion chromatography was used to examine the binding of a monoclonal anti-idiotopic antibody, F6, to its idiotope on the murine plasmacytoma IgA, TEPC-15. Chromatographic behavior revealed a strong association between T-15 and F6, which was previously characterized by solid-phase immunoassay as recognizing a nonbinding site epitope of the T-15. This chromatographic pattern suggests that the inability of the hapten phosphorylcholine to inhibit the anti-idiotope:idiotope relationship in solid-phase immunoassay might be equally explained by the low affinity of the hapten relative to the high affinity of the anti-idiotope antibody. Bivalent interactions between solid-phase IgA and liquid-phase IgG should enhance the binding of the anti-idiotope to an extent that would prevent the hapten from dissociating the complex. Under these solid-phase assay conditions, observation of hapten inhibition may, in some cases, indicate site specificity, but absence of inhibition cannot be interpreted. Computer simulations of solid-phase hapten inhibition scenarios were used to evaluate the qualitative nature of binding inhibition profiles expected under various conditions of liquid- and solid-phase reactant affinities and concentrations. The apparently unusual inhibition curves previously observed in the T-15:anti-T-15 studies in which the degree of binding inhibition by hapten appeared to be independent of anti-idiotope concentration may be predictable in cases of excess solid-phase epitope; the plateau inhibition value is a function of relative affinity constants and concentrations of solid-phase and inhibitor components. The results additionally suggest that the affinity of a liquid-phase antibody may modulate the effective concentration of solid-phase epitope.     

Formation of an infinite beta-sheet arrangement dominates the crystallization behavior of lambda-type antibody light chains

The packing interactions in crystals of human lambda-type antibody light chain dimers have been reviewed. These homologous proteins are composed of individually specific variable domains, but all have very similar constant domain sequences. The proteins do not emulate each other in their overall crystallization behavior: each attains an individually characteristic space group or unit cell dimensions. However, each of these protein crystals has one unit cell dimension in common, 72.4(+/- 0.2) A. Examination of the protein packing in these crystals reveals that the common cell dimension is a consequence of a packing arrangement of their constant domains, which is conserved in all three crystals. In this striking arrangement, beta-sheets of adjacent constant domains are placed in juxta-position to form an "infinite chain". Although this constant domain packing pattern is rigorously conserved, the variable domain packing arrangements in each of these crystals are different. The conservation of the "infinite" beta-sheet pattern suggests that the constant domain interactions dominate the thermodynamic energy of lattice formation, probably through a combination of specific hydrogen bond formations and by a decrease in the solvent-accessible surface. A single amino acid substitution prohibits this characteristic interneighbor hydrogen bond pattern in the homologous kappa-type light chains. This may explain the observation that very few kappa-type light chains have been crystallized.     

Precocene causes male determination in the aphid Myzus persicae

When adult apterous viviparous females of Myzus persicae, reared in short night conditions at 21–23°C, are treated with the precocene analogue 6-methoxy-7-ethoxy-2,2-dimethylchromene, they deposit males towards the end of their reproductive lives. The first-born normal daughters of the treated females also deposit some males at various times in their reproductive lives. Karyotypic analysis was used to investigate the sequence of male and female embryos in the ovarioles of precociously metamorphosed aphids. The experiments support the hypothesis (Mittler et al., 1979) that juvenile hormone level controls the sex determination process in aphids. Since male aphids have an XO sex chromosome constitution, this implies that juvenile hormone level influences the behaviour of the X-chromosomes at or before the single maturation division of the egg. At this division one X-chromosome is eliminated from eggs which will develop as males. Aphids provide the first example of a specific endocrine influence on chromosome behaviour in sex determination.     

Penetration of Bdellovibrio bacteriovorus into Host Cells

Electron microscopy reveals that, in Bdellovibrio infection, after the formation of a passage pore in the host cell wall, the differentiated parasite penetration pole is associated with the host protoplast. This firm contact persists throughout the parasite penetration and after this process is completed. In penetrated hosts this contact is also apparent by phase microscopy. The association between the walls of the parasite and the host at the passage pore, on the other hand, is transient. Bdellovibrio do not penetrate hosts whose protoplast and cell walls are separated by plasmolysis, or in which the membrane-wall relationship is affected by low turgor pressure. It is concluded, therefore, that for penetration to occur it is essential that the host protoplast be within reach of the parasite, so that a firm contact can be established between them. A penetration mechanism is proposed that is effected by forces generated by fluxes of water and solutes due to structural changes in the infected host envelope. These forces cause a differential expansion of the host protoplast and cell wall and their separation from each other around the entry site, while the parasite remains firmly anchored to the host protoplast. Consequently, the parasite ends up enclosed in the expanded host periplasm. The actual entry, therefore, is a passive act of the parasite.