Attraction Basins as Gauges of Robustness against Boundary Conditions in Biological Complex Systems

One fundamental concept in the context of biological systems on which researches have flourished in the past decade is that of the apparent robustness of these systems, i.e., their ability to resist to perturbations or constraints induced by external or boundary elements such as electromagnetic fields acting on neural networks, micro-RNAs acting on genetic networks and even hormone flows acting both on neural and genetic networks. Recent studies have shown the importance of addressing the question of the environmental robustness of biological networks such as neural and genetic networks. In some cases, external regulatory elements can be given a relevant formal representation by assimilating them to or modeling them by boundary conditions. This article presents a generic mathematical approach to understand the influence of boundary elements on the dynamics of regulation networks, considering their attraction basins as gauges of their robustness. The application of this method on a real genetic regulation network will point out a mathematical explanation of a biological phenomenon which has only been observed experimentally until now, namely the necessity of the presence of gibberellin for the flower of the plant Arabidopsis thaliana to develop normally

Positive feedback circuits and adaptive regulations in bacteria.

The mechanisms by which bacteria adapt to changes in their environment involve transcriptional regulation in which a transcriptional regulator responds to signal(s) from the environment and regulates (positively or negatively) the expression of several genes or operons. Some of these regulators exert a positive feedback on their own expression. This is a necessary (although not sufficient) condition for the occurrence of multistationarity. One biological consequence of multistationarity may be epigenetic modifications, a hypothesis unusual to microbiologists, in spite of some well-known epigenetic modifications in bacteria. We propose here that the occurrence of mucoidy in the opportunistic pathogen Pseudomonas aeruginosa, which is currently attributed to mutations only, may also be an epigenetic modification. A theoretical approach using a generalised logical analysis lends credit to this hypothesis and suggests experiments to ascertain it.Journal ArticleResearch Support, Non-U.S. Gov'tSCOPUS: ar.jinfo:eu-repo/semantics/publishe

Production of an extracellular milk-clotting activity during development in Myxococcus xanthus.

We describe here an extracellular proteolytic activity secreted during both growth and submerged development by Myxococcus xanthus DK1622. This activity yields the clotting of kappa-casein at pH 6 and is inhibited by specific inhibitors of aspartic proteases. Secretion of this milk-clotting proteolytic activity (of Mcp) is time regulated during the developmental cycle, with a large increase near 9 h poststarvation, but its production does not require cell-cell contact. The lack of secretion of this activity by several developmental mutants in submerged development conditions shows that Mcp production is developmentally regulated

A bioluminescence assay for screening thermoregulated genes in a psychrotrophic bacterium Pseudomonas fluorescens

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Bistability and hysteresis in epigenetic regulation of the lactose operon. Since Delbrück, a long series of ignored models.

International audienceBistability is the capacity of a system to switch in an "all-or-none" manner between alternative steady states. This powerful concept originates from the analysis of non-linear equations driving open systems. It is one of the various patterns of regulation associated with a particular class of dynamic structures that Glansdorff and Prigogine baptised "dissipative structures". The idea of discontinuous transitions between alternative states was first formulated much earlier, by Delbr? in 1949. Cohn and Horibata and Novick and Weiner confirmed that such transitions occur in experiments on the lactose operon carried out ten years later. Modelling with non-linear differential equations made it possible to simulate the dynamic behaviour of the lac operon, and modelling by asynchronous logical analysis elucidated the determinant role played by positive feedback circuits in the emergence of multistationarity. Nevertheless, these studies were largely ignored until the recent demonstration of the hysteretic nature of the bistable transition between alternative states of the lac operon. As originally suggested by Delbr? the pattern of lactose consumption adopted by the bacterium is controlled epigenetically rather than genetically: the true key determinant is the direction of change of an environmental variable with respect to the structural components of the operon

Temperature regulation of lipase secretion by Pseudomonas fluorescens strain MFO

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