Repression

Brent et al., 1989] Brent, G. A., Dunn, M. K., Harney, J. W., Gulick, T., and Larsen, P. R. Thyroid hormone aporeceptor represses Ta inducible promoters and blocks activity of the retinoic acid receptor. New Biol. 1 (1989) 329-336 [Cevc and Marsh, 1987] Cevc, G., and Marsh, D. Phospholipid Bilayers Physical Principles and Models. John Wiley Sons, New York, 1987. 

An excess of acetic acid is usually added before heating in order to repress the hydrolysis (and also the thermal dissociation) of the ammonium acetate, thus preventing the escape of ammonia. The excess of acetic acid, together with the water, is removed by slow fractional distillation. The method is rarely used except for the preparation of acetamide. 

The nitronium ions produced in this way tend to repress the selfdehydration of the nitric acid and therefore the net concentration of nitronium ions is not proportional to the concentration of the catalyst. When sufficient sulphuric acid has been added to make the self-ioniza-tion of nitric acid relatively unimportant, the nitronium ions will be produced predominantly from the above ionization, and the acceleration will follow a linear law. 

Chloroanisole and p-nitrophenol, the nitrations of which are susceptible to positive catalysis by nitrous acid, but from which the products are not prone to the oxidation which leads to autocatalysis, were the subjects of a more detailed investigation. With high concentrations of nitric acid and low concentrations of nitrous acid in acetic acid, jp-chloroanisole underwent nitration according to a zeroth-order rate law. The rate was repressed by the addition of a small concentration of nitrous acid according to the usual law rate = AQ(n-a[HN02]atoioh) -The nitration of p-nitrophenol under comparable conditions did not accord to a simple kinetic law, but nitrous acid was shown to anticatalyse the reaction. 

In a back titration, a slight excess of the metal salt solution must sometimes be added to yield the color of the metal-indicator complex. Where metal ions are easily hydrolyzed, the complexing agent is best added at a suitable, low pH and only when the metal is fully complexed is the pH adjusted upward to the value required for the back titration. In back titrations, solutions of the following metal ions are commonly employed Cu(II), Mg, Mn(II), Pb(II), Th(IV), and Zn. These solutions are usually prepared in the approximate strength desired from their nitrate salts (or the solution of the metal or its oxide or carbonate in nitric acid), and a minimum amount of acid is added to repress hydrolysis of the metal ion. The solutions are then standardized against an EDTA solution (or other chelon solution) of known strength. 

Eigure 6 enables a comparison to be made of kj a values in stirred bioreactors and bubble columns (51). It can be seen that bubble columns are at least as energy-efficient as stirred bioreactors in coalescing systems and considerably more so when coalescence is repressed at low specific power inputs (gas velocities). 

P/MForging. Even after conventional repressing of a P/M component, it is stiU difficult to increase density above 95%. However, hiU density in a P/M part improves its properties. Hot isostatic pressing in autoclaves works weU, especiaUy for titanium and superaUoy components, but the capital equipment is expensive and production rates are slow. 

In general, sizing, repressing, and coining are performed at room temperature. With elevated temperatures, a protective atmosphere must be provided. 

A few fireclay refractories are produced by the stiff-mud process using an auger machine that pugs, de-aks, and continuously extmdes a clay column. A wke cutter cuts the clay into blanks which are then sized, shaped, and branded by a repress machine. 

A number of the genes involved in the biosynthesis of thiamine in E. coli (89—92), i hium meliloti (93), B. suhtilis (94), and Schi saccharomycespomhe (95,96) have been mapped, cloned, sequenced, and associated with biosynthetic functions. Thiamine biosynthesis is tightly controlled by feedback and repression mechanisms limiting overproduction (97,98). A cost-effective bioprocess for production of thiamine will require significant additional progress. 

This is the reverse Pasteur or Crabtree effect and is also known as glucose inhibition or cataboHte repression. In the presence of higher sugar concentrations, synthesis of respiratory enzymes such as cytochromes is inhibited. 

BaS is leached from the black ash by hot water. The resultkig solution is filtered and then treated with soda ash or carbon dioxide or a combination of the two to precipitate fine BaCO crystals, which ki turn ate filtered and dried. Sulfide values can be recovered as H2S, NaHS, Na2S, or elemental sulfur. Carbon dioxide, detrimental to high BaS yields, is repressed according to the Boudouard equiUbtium... 

Two kinds of inflation can be considered general, or open, inflation and repressed, or differential, inflation. In the first case, all costs and... 

Microorganisms exhibit nutritional preferences. The enzymes for common substrates such as glucose are usually constitutive, as are the enzymes for common or essential metabohc pathways. Furthermore, the synthesis of enzymes for attack on less common substrates such as lactose is repressed by the presence of appreciable amounts of common substrates or metabolites. This is logical for cells to consei ve their resources for enzyme synthesis as long as their usual substrates are readily available. If presented with mixed substrates, those that are in the main metabolic pathways are consumed first, while the other substrates are consumed later after the common substrates are depleted. This results in diauxic behavior. A diauxic growth cui ve exhibits an intermediate growth plateau while the enzymes needed for the uncommon substrates are synthesized (see Fig. 24-2). There may also be preferences for the less common substrates such that a mixture shows a sequence of each being exhausted before the start of metabolism of the next. 

NOj ions/ Addition of water to nitric acid at first diminishes its electrical conductivity by repressing the autoprotolysis reactions mentioned above. For example, at -10° the conductivity decrea.ses from 3.67 x 10 ohm cm to a minimum of 1.08 x 10" ohm" cm at 1.75 molal H2O (82.8% NjOs) before rising again due to the increasing formation of the hydroxonium ion according to the acid-base equilibrium... 

Unterdruck-. low-pressure vacuum. unterdrUcken, v.t. suppress repress op-... 

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