Loss of unstably amplified dihydrofolate reductase genes from mouse cells is greatly accelerated by hydroxyurea

Previous work has shown that mammalian cells that carry unstably amplified genes for dihydrofolate reductase (DHFR) gradually lose the amplified DHFR genes when grown in the absence of the DHFR inhibitor methotrexate (MTX). Unstably amplified genes occur on small acentric chromosomes called double minutes (DMs) or even smaller chromatin fragments, in contrast to stably amplified genes, which reside in centromere-containing chromosomes. We have found that the rate of loss of the unstably amplified DHFR genes can be greatly increased by growing the cells in the presence of a nonlethal concentration of hydroxyurea. For example, in one MTX-resistant subline studied, approximate to 90% of the original DHFR gene dosage is lost in 25-30 cell doublings in the absence of MTX. The same degree of loss is achieved, however, in <4 doublings if cells are grown in the presence of 50 μM hydroxyurea. This new effect of hydroxyurea does not appear to be due to changes in plating efficiency or selective cytotoxicity. In particular, no increase in cell death occurs at 50 μM hydroxyurea, and cells continue to multiply, albeit 1/2 to 2/3 as fast as in the absence of hydroxyurea. The ability to selectively accelerate the loss of amplified genes from mammalian cells as shown in the present work may have important implications both for the problem of drug resistance in cancer chemotherapy and for curing mammalian cells of extrachromosomally maintained DNA genomes of pathogenic viruses

SINGLE SPECIES VERSUS MULTIPLE SPECIES MODELS: THE ECONOMIC IMPLICATIONS

Ecologists frequently note the importance of modelling entire ecosystems rather than single species, but most bioeconomic models in the current literature focus on a single species. While the mathematical difficulty of multiple species may quickly become overwhelming, sometimes making the single species option necessary, it is important to recognise the significance of the single species assumption to the model results. In this paper, the authors address the economic significance of this assumption through the development of a multiple species model and demonstrate the importance of interrelationships and economic values to the survival of endangered species.Resource /Energy Economics and Policy,

Random walks with thermalizing collisions in bounded regions; physical applications valid from the ballistic to diffusive regimes

The behavior of a spin undergoing Larmor precession in the presence of fluctuating fields is of interest to workers in many fields. The fluctuating fields cause frequency shifts and relaxation which are related to their power spectrum, which can be determined by taking the Fourier transform of the auto-correlation functions of the field fluctuations. Recently we have shown how to calculate these correlation functions for all values of mean free path (ballistic to diffusive motion) in finite bounded regions, using the model of persistent continuous time random walks (CTRW) for particles subject to scattering by fixed (frozen) scattering centers so that the speed of the moving particles is not changed by the collisions. In this work we show how scattering with energy exchange from an ensemble of scatterers in thermal equilibrium can be incorporated into the CTRW. We present results for 1,2 and 3 dimensions. The results agree for all these cases contrary to the previously studied 'frozen' models. Our results for the velocity autocorrelation function show a long time tail $\left( \sim t^{-1/2}\right)$, which we also obtain from conventional diffusion theory, with the same power, independent of dimensionality. Our results are valid for any Markovian scattering kernel as well as any kernel based on a scattering cross section $\sim1/v.$Comment: 43 pages, 12 figure

Discrimination and Quantification of Glomerular Receptor Subtypes for Atrial Natriuretic Factor (Anf)

Binding sites for atrial natriuretic factor (ANF) were determined on isolated rat glomeruli as well as on glomerular membranes. To define optimal conditions, binding of ANF was investigated varying incubation time, temperature and protein concentration. Binding conditions were found to be best at 4°C for 5 hours with 15 μg of glomerular protein. Saturation and affinity cross-linking experiments confirmed the presence of two distinct receptor subtypes - the B-receptor (130 kDa) and the C-receptor (65 kDa). Quantitative differentiation of both ANF binding sites was achieved by competitive displacement with two different unlabeled ANF ligands: a) rANF(99-126) (homologous displacement), b) des(18-22)rANF(4-23)NH2(heterologous displacement). Intact glomeruli and glomerular membranes did not differ significantly in receptor density for the B-receptor (71 ± 37 vs. 94 ± 53 fmol/mg protein) or the C-receptor (976 ± 282 vs. 966 ± 167 fmol/mg protein) or in affinity constants for the B-receptor (43 ± 36 vs. 52 ± 44 pM) or the C-receptor (876 ± 377 vs. 307 ± 36 pM). Glomerular membranes compared to glomeruli showed less nonspecific binding and less intra-assay variation of measuring points done in triplicates. This method of selective displacement should allow to study the influence of various physiological and pathophysiological conditions on the binding properties of B-and C-receptors for ANF

Under-dominance constrains the evolution of negative autoregulation in diploids

Regulatory networks have evolved to allow gene expression to rapidly track changes in the environment as well as to buffer perturbations and maintain cellular homeostasis in the absence of change. Theoretical work and empirical investigation in Escherichia coli have shown that negative autoregulation confers both rapid response times and reduced intrinsic noise, which is reflected in the fact that almost half of Escherichia coli transcription factors are negatively autoregulated. However, negative autoregulation is exceedingly rare amongst the transcription factors of Saccharomyces cerevisiae. This difference is all the more surprising because E. coli and S. cerevisiae otherwise have remarkably similar profiles of network motifs. In this study we first show that regulatory interactions amongst the transcription factors of Drosophila melanogaster and humans have a similar dearth of negative autoregulation to that seen in S. cerevisiae. We then present a model demonstrating that this fundamental difference in the noise reduction strategies used amongst species can be explained by constraints on the evolution of negative autoregulation in diploids. We show that regulatory interactions between pairs of homologous genes within the same cell can lead to under-dominance - mutations which result in stronger autoregulation, and decrease noise in homozygotes, paradoxically can cause increased noise in heterozygotes. This severely limits a diploid's ability to evolve negative autoregulation as a noise reduction mechanism. Our work offers a simple and general explanation for a previously unexplained difference between the regulatory architectures of E. coli and yeast, Drosophila and humans. It also demonstrates that the effects of diploidy in gene networks can have counter-intuitive consequences that may profoundly influence the course of evolution

A Diffusion Approximation to the Markov Chains Model of the Financial Market and the Expected Riskless Profit Under Selling of Call and Put Options

A discrete time model of financial markets is considered. It is assumed that the stock price evolution is described by a homogeneous Markov chain. In the focus of attention is the expected value of the guaranteed profit of the investor that arises when the jumps of the stock price are bounded. The suggested diffusion approximation for the Markov chain allows establishing a convenient approximate formula for the studied characteristic.Ergodic and irreducible Markov chains, Stationary distribution, Local limit theorem, Upper hedge, Upper rational price

A Diffusion Approximation for the Riskless Profit Under Selling of Discrete Time Call Options. Non-identically Distributed Jumps

A discrete time model of financial markets is considered. It is assumed that the relative jumps of the risky security price are independent non-identically distributed random variables. In the focus of attention is the expected non-risky profit of the investor that arises when the jumps of the stock price are bounded while the investor follows the upper hedge. The considered discrete time model is approximated by a continuous time model that generalizes the classical geometrical Brownian motion.Asymptotic uniformity, Local limit theorem, Volatility