The ornithine cycle in Neurospora and its genetic control

It has been emphasized by Haldane (1) that for studies of intermediary metabolism "the new science of genetics furnishes a very powerful method." Such a method is founded upon the general premises that genes control many of the chemical reactions within an organism, and that gene mutations by blocking a reaction chain at various points may, in effect, resolve a metabolic process into some of its constituent stages. For instance, the genetics of such diseases as alcaptonuria and cystinuria have elucidated certain problems in human metabolic processes (2), and studies in the genetics of plant pigments have increased the knowledge of of the biochemistry of anthocyanins (3). But the study of metabolism by way of genetic differences in naturally occurring populations is limited not only by the low rate of mutation but also by the lethal character of most mutations of genes controlling vital functions. By increasing the mutation rate of an organism, through irradiation or otherwise, it is possible to create a number of genetic blocks at various steps in the syntheses of substances or in other processes of metabolism. The problem of preserving mutations ordinarily lethal has been met by Beadle and Tatum (4) in a general course of procedure developed around work with the ascomycetous mold Neurospora. The wild type of this organism is able to carry out all the syntheses essential to its normal growth and reproduction if biotin, inorganic salts, and a suitable source of carbon are available. Strains of Neurospora are irradiated with x-ray or ultraviolet rays on the assumption that mutations will be induced in genes controlling the syntheses of such substances as vitamins and amino acids. Mutant strains of this kind cannot grow on merely inorganic salts, sugar, and biotin, "minimal medium," but can be expected to grow if the product of the blocked synthesis is added to the minimal medium. From irradiated Neurospora there has been isolated in this laboratory a series of mutant strains which require for growth the presence of arginine in the culture medium. A study of the specific biochemical characteristics of members of this group of mutants has made it possible to demonstrate in Neurospora crassa an ornithine cycle similar to that proposed by Krebs and Henseleit (5) as occurring in mammalian liver, and to assign various steps in the cycle to the influence of particular single genes. To our knowledge the ornithine cycle has not previously been demonstrated in plants

Growth inhibition of neurospora by canavanine, and its reversal

Canavanine, an amino acid from jack beans, was discovered by Kitagawa and coworkers in 1929 (1, 2). The substance is not combined in the proteins of the seed, but occurs in the free state, and makes up 2.5 per cent of the dry weight of jack beans (3). In a series of papers available to the authors for the most part in abstract only, the Japanese workers have reported extensive investigations into the chemistry and physiology of the substance. The structure of canavanine was established by Gulland and Morris (4) and by Kitagawa and Takani (5) as NH2•C(:NII)•NII•O•CH2•CH2•CHNH2•COOH. Natural canavanine is of the L configuration (6)

Superconductivity of SrTiO_{3-\delta}

Superconducting SrTiO_{3-\delta} was obtained by annealing single crystalline SrTiO_3 samples in ultra high vacuum. An analysis of the V(I) characteristics revealed very small critical currents I_c which can be traced back to a unavoidable doping inhomogeneity. R(T) curves were measured for a range of magnetic fields B at I<<I_c, thereby probing only the sample regions with the highest doping level. The resulting curves B_{c2}(T) show upward curvature, both at small and strong doping. These results are discussed in the context of bipolaronic and conventional superconductivity with Fermi surface anisotropy. We conclude that the special superconducting properties of SrTiO_{3-\delta} can be related to its Fermi surface and compare this finding with properties of the recently discovered superconductor MgB_2.Comment: EPJ style, 6 pages, 8 figures; minor changes, Fig. 5 replaced; use PDF version for printout

Nonlinearity without Superluminality

Quantum theory is compatible with special relativity. In particular, though measurements on entangled systems are correlated in a way that cannot be reproduced by local hidden variables, they cannot be used for superluminal signalling. As Czachor, Gisin and Polchinski pointed out, this is not true for general nonlinear modifications of the Schroedinger equation. Excluding superluminal signalling has thus been taken to rule out most nonlinear versions of quantum theory. The no superluminal signalling constraint has also been used for alternative derivations of the optimal fidelities attainable for imperfect quantum cloning and other operations. These results apply to theories satisfying the rule that their predictions for widely separated and slowly moving entangled systems can be approximated by non-relativistic equations of motion with respect to a preferred time coordinate. This paper describes a natural way in which this rule might fail to hold. In particular, it is shown that quantum readout devices which display the values of localised pure states need not allow superluminal signalling, provided that the devices display the values of the states of entangled subsystems as defined in a non-standard, but natural, way. It follows that any locally defined nonlinear evolution of pure states can be made consistent with Minkowski causality.Comment: References update

The shadow banking system: implications for fi nancial regulation

The current financial crisis has highlighted the changing role of financial institutions and the growing importance of the “shadow banking system” that grew on the back of the securitisation of assets and the integration of banking with capital market developments. This trend has been most pronounced in the United States, but has had a profound influence for the global financial system as a whole. In a market-based financial system, banking and capital market developments are inseparable, and funding conditions are closely tied to the fluctuations of leverage of market-based fi nancial intermediaries. Balance sheet growth of market-based financial intermediaries provides a window on liquidity in the sense of availability of credit, while contractions of balance sheets have tended to precede the onset of financial crises. Securitisation was intended as a way to disperse credit risk to those who were better able to absorb losses, but instead securitisation served to increase the fragility of the financial system as a whole by allowing banks and other intermediaries to leverage up by buying each others’ securities. In the new, post-crisis financial system, the role of securitisation is likely to be held in check by more stringent financial regulation and the recognition of the importance of preventing excessive leverage and maturity mismatch in undermining financial stability.

Liquidity and financial contagion.

There is an apparent puzzle at the heart of the 2007 credit crisis. The subprime mortgage sector is small relative to the financial system as a whole and the exposure was widely dispersed through securitization. Yet the crisis in the credit market has been potent. Traditionally, financial contagion has been viewed through the lens of defaults, where if A has borrowed from B and B has borrowed from C, then the default of A impacts B, which then impacts C, etc. However, in a modern market-based financial system, the channel of contagion is through price changes and the measured risks and marked-to-market capital of financial institutions. When balance sheets are marked to market, asset price changes show up immediately on balance sheets and elicit response from financial market participants. Even if exposures are dispersed widely throughout the financial system, the potential impact of a shock can be amplified many-fold through market price changes.