The Cell Density Factor CMF Regulates the Chemoattractant Receptor cAR1 in Dictyostelium

Starving Dictyostelium cells aggregate by chemotaxis to cAMP when a secreted protein called conditioned medium factor (CMF) reaches a threshold concentration. Cells expressing CMF antisense mRNA fail to aggregate and do not transduce signals from the cAMP receptor. Signal transduction and aggregation are restored by adding recombinant CMF. We show here that two other cAMP-induced events, the formation of a slow dissociating form of the cAMP receptor and the loss of ligand binding, which is the first step of ligand-induced receptor sequestration, also require CMF. Vegetative cells have very few CMF and cAMP receptors, while starved cells possess ~40,000 receptors for CMF and cAMP. Transformants overexpressing the cAMP receptor gene cAR1 show a 10-fold increase of [3H]cAMP binding and a similar increase of [125I]CMF binding; disruption of the cAR1 gene abolishes both cAMP and CMF binding. In wild-type cells, downregulation of cAR1 with high levels of cAMP also downregulates CMF binding, and CMF similarly downregulates cAMP and CMF binding. This suggests that the cAMP binding and CMF binding are closely linked. Binding of ~200 molecules of CMF to starved cells affects the affinity of the majority of the cAR1 cAMP receptors within 2 min, indicating that an amplifying mechanism allows one activated CMF receptor to regulate many cARs. In cells lacking the G-protein β subunit, cAMP induces a loss of cAMP binding, but not CMF binding, while CMF induces a reduction of CMF binding without affecting cAMP binding, suggesting that the linkage of the cell density-sensing CMF receptor and the chemoattractant cAMP receptor is through a G-protein.

The terminal redundancy of the retrovirus genome facilitates chain elongation by reverse transcriptase

Transcription of DNA from the RNA genome of retroviruses by reverse transcriptase involves an unusual translocation of the growing chain from the 5' end to the 3' end of the RNA template. In order to elucidate the mechanism by which this translocation occurs, we have used chain termination to analyze nascent viral DNA synthesized in vitro by avian sarcoma virus, and we have determined the nucleotide sequence of appropriate regions of viral DNA isolated from infected cells and cloned into prokaryotic vectors. Our results provide direct experimental evidence for a previously proposed model in which a short terminal redundancy in viral RNA, and a DNA copy of the redundant sequence, are used to allow the growing DNA chain to move from the 5' to the 3' end of the template. Transcription of avian sarcoma virus RNA with purified reverse transcriptase also generates an anomalous product, a hairpin DNA that arises when the initial DNA transcript folds back on itself to continue synthesis. The foldback is mediated by an inverted repeat of 5 nucleotides in the sequence of nascent DNA. Anomalous hairpin DNA is not produced by detergent-activated virions. Thus, constituents of the virons or the configuration of encapsidated viral RNA must faciliatate correct transcription

Autopoiesis in creativity and art

The term autopoiesis, (meaning ‘self’) and ‘poiesis’ (mean- ing ‘creation, production’) defines a system capable of repro- ducing and maintaining itself. The term was introduced by the theoretical biologists, Humberto Maturana and Francisco Varela, in 1972 to define the self-maintaining chemistry of living cells. The term has subsequently also been applied to the fields of systems theory and sociology. In this paper we apply this model to characterise creativity in art practise

Bose-Einstein condensation and superfluidity of dilute Bose gas in a random potential

We develop the dilute Bose gas model with random potential in order to understand the Bose system in random media such as 4He in porous glass. Using the random potential taking account of the pore size dependence, we can compare quantitatively the calculated specific heat with the experimental results, without free parameters. The agreement is excellent at low temperatures, which justifies our model. The relation between Bose condensation and superfluidity is discussed. Our model can predict some unobserved phenomena in this system.Comment: 9 pages, 11 figures, accepted for publication in Phys. Rev.

Transduction of a cellular oncogene: The genesis of Rous sarcoma virus

The oncogene of Rous sarcoma virus (v-src) arose by transduction of a cellular gene (c-src). In an effort to explore the mechanism of transduction, we have identified the splice acceptor site used in the genesis of mRNA for v-src, shown that an equivalent site is used in the splicing of mRNA for c-src, and determined the nucleotide sequence from the boundaries of homology between v-src and c-src. Our data indicate that (i) only a portion of c-src is represented within v-src, (ii) the leftward recombination between the genome of the transducing virus and c-src occurred in an intron of the cellular gene, (iii) v-src is in part a spliced version of the corresponding portion of c-src, and (iv) nucleotide sequences represented once in the genome of the transducing virus become duplicated to flank v-src. These findings indicate that the first step in transduction is probably recombination between DNA forms of the transducing viral genome and c-src and otherwise support the prevailing model for transduction by retroviruses. The carboxyl termini of the proteins encoded by v-src and c-src differ appreciably. An unidentified domain of 127 or 128 nucleotides is located at different positions in the genomes of two strains of RSV and gives evidence of being a foreign element that entered the viral genomes by genetic transposition independent of the transduction of src

Well-Posed Initial-Boundary Evolution in General Relativity

Maximally dissipative boundary conditions are applied to the initial-boundary value problem for Einstein's equations in harmonic coordinates to show that it is well-posed for homogeneous boundary data and for boundary data that is small in a linearized sense. The method is implemented as a nonlinear evolution code which satisfies convergence tests in the nonlinear regime and is robustly stable in the weak field regime. A linearized version has been stably matched to a characteristic code to compute the gravitational waveform radiated to infinity.Comment: 5 pages, 6 figures; added another convergence plot to Fig. 2 + minor change

Instability of generalised AdS black holes and thermal field theory

We study black holes in AdS-like spacetimes, with the horizon given by an arbitrary positive curvature Einstein metric. A criterion for classical instability of such black holes is found in the large and small black hole limits. Examples of large unstable black holes have a B\"ohm metric as the horizon. These, classically unstable, large black holes are locally thermodynamically stable. The gravitational instability has a dual description, for example by using the $AdS_7 \times S^4$ version of the AdS/CFT correspondence. The instability corresponds to a critical temperature of the dual thermal field theory defined on a curved background.Comment: 1+16 pages. 1 figure. LaTeX. Minor clarification

Foliations of Isonergy Surfaces and Singularities of Curves

It is well known that changes in the Liouville foliations of the isoenergy surfaces of an integrable system imply that the bifurcation set has singularities at the corresponding energy level. We formulate certain genericity assumptions for two degrees of freedom integrable systems and we prove the opposite statement: the essential critical points of the bifurcation set appear only if the Liouville foliations of the isoenergy surfaces change at the corresponding energy levels. Along the proof, we give full classification of the structure of the isoenergy surfaces near the critical set under our genericity assumptions and we give their complete list using Fomenko graphs. This may be viewed as a step towards completing the Smale program for relating the energy surfaces foliation structure to singularities of the momentum mappings for non-degenerate integrable two degrees of freedom systems.Comment: 30 pages, 19 figure

Dynamical electron transport through a nanoelectromechanical wire in a magnetic field

We investigate dynamical transport properties of interacting electrons moving in a vibrating nanoelectromechanical wire in a magnetic field. We have built an exactly solvable model in which electric current and mechanical oscillation are treated fully quantum mechanically on an equal footing. Quantum mechanically fluctuating Aharonov-Bohm phases obtained by the electrons cause nontrivial contribution to mechanical vibration and electrical conduction of the wire. We demonstrate our theory by calculating the admittance of the wire which are influenced by the multiple interplay between the mechanical and the electrical energy scales, magnetic field strength, and the electron-electron interaction