Search

A computer model is described which is capable of predicting changes in cell composition, cell size, cell shape, and the timing of chromosome synthesis in response to changes in external glucose limitation. The model is constructed primarily from information on unrestricted growth in glucose minimal medium. The ability of the model to make reasonable quantitative predictions under glucose-limitation is a test of the plausibility of the basic biochemical mechanisms included in the model. Such a model should be of use in differentiating among competing hypotheses for biological mechanisms and in suggesting as yet unobserved phenomena. The last two points are illustrated with the testing of a mechanism for the control of the initiation of DNA synthesis and predictions on cell-width variations during the division cycle.

Discontinuous precipitation in an austenitic Fe-Mn-V-C alloy has been studied using transmission electron microscopy of extraction replicas and thin foils. The vanadium carbide precipitates formed during the discontinuous reaction are present either as particles or as long fibres. The coincidence site lattice model for high-angle grain boundaries has been used to relate each precipitate morphology to the nature of the grain boundary at which it forms. Fibrous precipitation occurs behind those boundaries in which there is a comparatively high density of coincidence sites, whereas particulate precipitation occurs at low coincidence boundaries.

A ledge mechanism of growth of colonies of discontinuous vanadium carbide precipitate in an austenitic Fe-Mn-V-C alloy has been established by observing the progress of the reaction using scanning electron microscopy. The mobility of the risers of the ledges was measured and found to be at least an order of magnitude higher than that of the broad faces of the ledges. An explanation of this difference in mobility involving the nucleation of vanadium carbide has been proposed. Transmission electron microscopy of a ledged colony boundary confirmed that the planar density of coincident sites on the risers of ledges, where fibrous precipitation occurs, was greater than that on the broad faces at which particulate precipitation takes place.

The orientation relationship between ferrite and precipitate particles of the hexagonal Laves phase, Fe2Nb, has been determined using electron diffraction techniques. There exists a single orientation relationship, which may be stated as:A model of the interface between the two phases is developed on the basis of this orientation relationship and the observed morphology and habit of the precipitate particles are discussed in terms of this model.

The deformation of two industrial alloys which exhibit superplasticity (Cu-9·5Al-4Fe and Cu-0·4Co-2·8Al-1·8Si) has been examined by subjecting thin circular diaphragms to one-sided hydrostatic pressure at elevated temperatures. The experimentally determined values of thinning factor (defined as the ratio of the actual thickness to the average thickness) for hemispherical domes show good agreement with theoretical predictions when the value of the strain-rate sensitivity is between 0·35 and 0·40. The various combinations of temperature, pressure, and forming time for optimum behaviour have been explored. An attempt has been made to correlate tensile data with the bulge-forming characteristics.

The process of intergranular fatigue cavitation at 500°C in a copper-base alloy (CDA 638) has been studied in low-frequency tension/compression cycling. The focus has been upon the role of grain size over the range 16–330 μm average dia. Specimens were also cut transverse and parallel to the rolling direction to investigate the influence of anisotropy. Fatigue life seems to be determined by a damage parameter based upon crack length at coarse grain sizes (50–330 μm) and upon void linkage at fine grain sizes (16–50 μm). The idea that failure is largely controlled by the ease of final crack propagation is emphasized by the anisotropic behaviour-material cut transverse to the rolling direction exhibited a markedly reduced life owing to the coplanarity of boundaries in the rolling direction which facilitated linkage.

A mutant strain of Escherichia colicapable of growth on l-1,2-propanediol was isolated previously. The mutant is characterized by constitutive production of a propanediol:nicotinamide adenenine dinucleotide (NAD) oxidoreductase which is essential for the new growth property. In the present study, it is shown that phage P1 cotransduces the genetic locus conferring this property and the genes for the utilization of l-fucose. A further indication of a relationship between these two growth properties is provided by the observation that wild-type E. coliexcretes propanediol during fermentation of l-fucose. Under these conditions, a propanediol dehydrogenase (lactaldehyde reductase) is induced. This enzyme migrates on diethylaminoethyl-cellulose with the propanediol dehydrogenase produced constitutively by the mutant strain. A key event in the establishment of the ability to grow on propanediol is evidently a shift in the expression and function of propanediol dehydrogenase; an enzyme catalyzing formation of a reduced fermentation product anaerobically in wild-type cells functions aerobically to oxidize this same product in the mutant. l-Lactaldehyde, which is thus derived from propanediol, is converted to l-lactate by another dehydrogenase (l-lactaldehyde:NAD oxidoreductase) which is constitutively produced by both wild-type and mutant cells. The normal function of this enzyme is not yet established. l-Lactate is converted to pyruvate by an inducible NAD-independent l-lactate dehydrogenase. Thus, the carbons of propanediol are brought into the central metabolic network of the cell.