An Application of the Computable General Equilibrium Model to Analyze U.S. Ariculture

The effects of exchange rate and capital stock changes are analyzed using a CGE model for the United States. The model is in the Walrasion tradition and is calibrated to 1982 data. Results indicate that a devaluation of the U.S. dollar has a positive effect on the agricultural sector and balance of trade, but has a negative effect on consumers

Pulsed Feedback Defers Cellular Differentiation

Environmental signals induce diverse cellular differentiation programs. In certain systems, cells defer differentiation for extended time periods after the signal appears, proliferating through multiple rounds of cell division before committing to a new fate. How can cells set a deferral time much longer than the cell cycle? Here we study Bacillus subtilis cells that respond to sudden nutrient limitation with multiple rounds of growth and division before differentiating into spores. A well-characterized genetic circuit controls the concentration and phosphorylation of the master regulator Spo0A, which rises to a critical concentration to initiate sporulation. However, it remains unclear how this circuit enables cells to defer sporulation for multiple cell cycles. Using quantitative time-lapse fluorescence microscopy of Spo0A dynamics in individual cells, we observed pulses of Spo0A phosphorylation at a characteristic cell cycle phase. Pulse amplitudes grew systematically and cell-autonomously over multiple cell cycles leading up to sporulation. This pulse growth required a key positive feedback loop involving the sporulation kinases, without which the deferral of sporulation became ultrasensitive to kinase expression. Thus, deferral is controlled by a pulsed positive feedback loop in which kinase expression is activated by pulses of Spo0A phosphorylation. This pulsed positive feedback architecture provides a more robust mechanism for setting deferral times than constitutive kinase expression. Finally, using mathematical modeling, we show how pulsing and time delays together enable “polyphasic” positive feedback, in which different parts of a feedback loop are active at different times. Polyphasic feedback can enable more accurate tuning of long deferral times. Together, these results suggest that Bacillus subtilis uses a pulsed positive feedback loop to implement a “timer” that operates over timescales much longer than a cell cycle

Long-term results of 32-mm alumina-on-alumina THA for avascular necrosis of the femoral head

BACKGROUND: Ceramic bearings in total hip arthroplasty (THA) have been introduced in clinical practice to minimize the problem of polyethylene particle-induced osteolysis. The aim of the study is to report the results of 68 consecutive alumina-on-alumina THAs done in 61 patients for avascular necrosis (AVN) of the femoral head. MATERIALS AND METHODS: In all implants a press-fit cup was used; it was combined with a 32-mm alumina head and with titanium-alloy stems. The mean age at surgery was 50 years. At an average follow-up of 13 years two hips have been revised, one for periprosthetic infection and one for excessive abduction of the cup. RESULTS: No revision for aseptic loosening is recorded; one anatomical cementless femoral stem had radiological evidence of definite aseptic loosening. No dislocations occurred, and no osteolysis was observed. CONCLUSIONS: The results support the application of alumina-alumina THA for long-lasting replacements

Fluctuations in spo0A Transcription Control Rare Developmental Transitions in Bacillus subtilis

Phosphorylated Spo0A is a master regulator of stationary phase development in the model bacterium Bacillus subtilis, controlling the formation of spores, biofilms, and cells competent for transformation. We have monitored the rate of transcription of the spo0A gene during growth in sporulation medium using promoter fusions to firefly luciferase. This rate increases sharply during transient diauxie-like pauses in growth rate and then declines as growth resumes. In contrast, the rate of transcription of an rRNA gene decreases and increases in parallel with the growth rate, as expected for stable RNA synthesis. The growth pause-dependent bursts of spo0A transcription, which reflect the activity of the spo0A vegetative promoter, are largely independent of all known regulators of spo0A transcription. Evidence is offered in support of a “passive regulation” model in which RNA polymerase stops transcribing rRNA genes during growth pauses, thus becoming available for the transcription of spo0A. We show that the bursts are followed by the production of phosphorylated Spo0A, and we propose that they represent initial responses to stress that bring the average cell closer to the thresholds for transition to bimodally expressed developmental responses. Measurement of the numbers of cells expressing a competence marker before and after the bursts supports this hypothesis. In the absence of ppGpp, the increase in spo0A transcription that accompanies the entrance to stationary phase is delayed and sporulation is markedly diminished. In spite of this, our data contradicts the hypothesis that sporulation is initiated when a ppGpp-induced depression of the GTP pool relieves repression by CodY. We suggest that, while the programmed induction of sporulation that occurs in stationary phase is apparently provoked by increased flux through the phosphorelay, bet-hedging stochastic transitions to at least competence are induced by bursts in transcription

Vapour phase catalytic transformations of 3-carene over chromium impregnated zeolites: Influence of organic bases

162-168Large pore zeolites, 13X and Y and medium pore zeolite ZSM-5 were synthesised by hydrothermal method, partially converted them into H-form by ion exchange. Each was impregnated with 2% Cr and they were characterised by physicochemical analysis

Influence of coke on the aromatization of 3-carene in the vapour phase over zeolites

264-269Conversions of 3-carene to cymenes over Na13X, NaY and NaZSM-5 are less than over NaH13X (29% proton exchange), NaHY (33% proton exchange) and NaHZSM-5 (35% proton exchange). In the presence of oxygen, at its different partial pressures, the formation of cymenes increases significantly with increase in time on stream, indicating the formations of some species in the coke during the reaction which enhances the activity of the catalysts. To identify the species that cause enhancement of the activity, zeolite NaHY (catalyst B) has been doped with different amounts of NaOH and the reactions have been carried out over them at 350°C, at a contact time of 1.0 h (contact time is expressed as the reciprocal of Whsv, the weight hourly space velocity representing the weight of 3-carene passed over 1 g of catalyst per hour). The coke deposited has been examined by IR, ESR and XRD. Formation of quinone type of carbonyl group, responsible for the enhanced activity of the zeolites is observed. The participation of the quinone group in the dehydrogenation process is confirmed by extracting the coke after the reaction and the same reaction is carried out over the extracted coke both in the presence and in the absence of oxygen under the same experimental conditions. The quinone group is present in an atmosphere of oxygen but is absent if there is no oxygen

Isomorphous substitution of chromium in AlPO-11 and AlPO-5 framework and their applications in the aromatization of 3-carene

605-609CrAPO-11 and CrAPO-5 have been synthesized hydrothermally using aluminium isopropoxide, H3PO4, Cr(NO3)3 and di-n-propylamine for CrAPO-11 and triethylamine for CrAPO-5. The isomorphous substitution of Cr in AlPO-11 and AlPO-5 framework has been confirmed by XRD, SEM, DRS. MAS NMR, TGA, pore-volume and chemical analysis. Formation of cymenes (p-cymene and m-cymene) from 3-carene is higher over reduced than that over oxidized catalysts. The higher plm (p-cymene/ m-cymene) ratio, observed over reduced than over oxidized catalysts shows that p-cymene does not isomerise to m-cymene. The poisoning effects of pyridine and 4-methylquinoline are the same over large pore CrAPO-5 but different over medium pore CrAPO-11