Monitoring of nutrient limitation in growing E. coli: A mathematical model of a ppGpp-based biosensor

Background: E. coli can be used as bacterial cell factories for production of biofuels and other useful compounds. The efficient production of the desired products requires careful monitoring of growth conditions and the optimization of metabolic fluxes. To avoid nutrient depletion and maximize product yields we suggest using a natural mechanism for sensing nutrient limitation, related to biosynthesis of an intracellular messenger - guanosine tetraphosphate (ppGpp). Results: We propose a design for a biosensor, which monitors changes in the intracellular concentration of ppGpp by coupling it to a fluorescent output. We used mathematical modelling to analyse the intracellular dynamics of ppGpp, its fluorescent reporter, and cell growth in normal and fatty acid-producing E. coli lines. The model integrates existing mechanisms of ppGpp regulation and predicts the biosensor response to changes in nutrient state. In particular, the model predicts that excessive stimulation of fatty acid production depletes fatty acid intermediates, downregulates growth and increases the levels of ppGpp-related fluorescence. Conclusions: Our analysis demonstrates that the ppGpp sensor can be used for early detection of nutrient limitation during cell growth and for testing productivity of engineered lines

Mathematical modelling of diurnal regulation of carbohydrate allocation by osmo-related processes in plants

Copyright & Usage © 2015 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.Peer reviewedPublisher PD

A mathematical model of bacteria capable of complete oxidation of ammonium predicts improved nitrogen removal and reduced production of nitrous oxide

The removal of excess nutrients from water ecosystems requires oxidation of toxic ammonium by two types of bacteria; one oxidizes ammonium to nitrite and the other oxidizes nitrite to nitrate. The oxidation of ammonium is often incomplete and nitrite accumulates. Nitrite is also toxic, and is converted by the ammoniumoxidizing bacteria to nitrous oxide, a powerful greenhouse gas. Here we use mathematical modeling to analyze a potential solution to the problems related to incomplete oxidation of ammonium. We propose that a single engineered nitrifying bacterium should be capable of complete oxidation of high concentrations of ammonium to nitrate. Our model is based on available data on ammonium- and nitrite-oxidizing bacteria. The model predicts that insertion of highly expressed genes of a nitrite oxidation system into the genome of an ammonia-oxidation bacterium should result in complete oxidation of ammonium to nitrate in nutrient-overloaded conditions. Due to its increased capacity to fully oxidize ammonium to nitrate, the proposed bacterium would display dramatically reduced production of nitrous oxide, and therefore might have great potential to reduce the greenhouse effect of nutrient-overloaded water system

Modelling the widespread effects of TOC1 signalling on the plant circadian clock and its outputs

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This work was supported by the European Commission FP7 Collaborative Project TiMet (project 245143). SynthSys is a Centre for Integrative and Systems Biology supported by BBSRC and EPSRC award D019621. Work in P.M. laboratory is supported by grants from the Ramón Areces Foundation, from the Spanish Ministry of Science and Innovation (MICINN) (BIO2010-16483) and from EUROHORCS (European Heads Of Research Councils) and the European Science Foundation (ESF) through the EURYI Award.Peer reviewedPublisher PD

Mathematical model of a serine integrase-controlled toggle switch with a single input

Dual-state genetic switches that can change their state in response to input signals can be used in synthetic biology to encode memory and control gene expression. A transcriptional toggle switch (TTS), with two mutually repressing transcription regulators, was previously used for switching between two expression states. In other studies, serine integrases have been used to control DNA inversion switches that can alternate between two different states. Both of these switches use two different inputs to switch ON or OFF. Here, we use mathematical modelling to design a robust one-input binary switch, which combines a TTS with a DNA inversion switch. This combined circuit switches between the two states every time it receives a pulse of a single-input signal. The robustness of the switch is based on the bistability of its TTS, while integrase recombination allows single-input control. Unidirectional integrase-RDF-mediated recombination is provided by a recently developed integrase-RDF fusion protein. We show that the switch is stable against parameter variations and molecular noise, making it a promising candidate for further use as a basic element of binary counting devices

Adjustment of carbon fluxes to light conditions regulates the daily turnover of starch in plants : a computational model

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Correction: Computational Model Explains High Activity and Rapid Cycling of Rho GTPases within Protein Complexes

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A single-input binary counting module based on serine integrase site-specific recombination

A device that counts and records the number of events experienced by an individual cell could have many uses in experimental biology and biotechnology. Here, we report a DNA-based ‘latch’ that switches between two states upon each exposure to a repeated stimulus. The key component of the latch is a DNA segment whose orientation is inverted by the actions of ϕC31 integrase and its recombination directionality factor (RDF). Integrase expression is regulated by an external input, while RDF expression is controlled by the state of the latch, such that the orientation of the invertible segment switches efficiently each time the device receives an input pulse. Recombination occurs over a time scale of minutes after initiation of integrase expression. The latch requires a delay circuit, implemented with a transcriptional repressor expressed in only one state, to ensure that each input pulse results in only one inversion of the DNA segment. Development and optimization of the latch in living cells was driven by mathematical modelling of the recombination reactions and gene expression regulated by the switch. We discuss how N latches built with orthogonal site-specific recombination systems could be chained together to form a binary ripple counter that could count to 2N − 1

The mechanism of ϕC31 integrase directionality : experimental analysis and computational modelling

Serine integrases, DNA site-specific recombinases used by bacteriophages for integration and excision of their DNA to and from their host genomes, are increasingly being used as tools for programmed rearrangements of DNA molecules for biotechnology and synthetic biology. A useful feature of serine integrases is the simple regulation and unidirectionality of their reactions. Recombination between the phage attP and host attB sites is promoted by the serine integrase alone, giving recombinant attL and attR sites, whereas the 'reverse' reaction (between attL and attR) requires an additional protein, the recombination directionality factor (RDF). Here, we present new experimental data on the kinetics and regulation of recombination reactions mediated by ϕC31 integrase and its RDF, and use these data as the basis for a mathematical model of the reactions. The model accounts for the unidirectionality of the attP × attB and attL × attR reactions by hypothesizing the formation of structurally distinct, kinetically stable integrase-DNA product complexes, dependent on the presence or absence of RDF. The model accounts for all the available experimental data, and predicts how mutations of the proteins or alterations of reaction conditions might increase the conversion efficiency of recombination

Quantitative analysis of regulatory flexibility under changing environmental conditions

The circadian clock controls 24-h rhythms in many biological processes, allowing appropriate timing of biological rhythms relative to dawn and dusk. Known clock circuits include multiple, interlocked feedback loops. Theory suggested that multiple loops contribute the flexibility for molecular rhythms to track multiple phases of the external cycle. Clear dawn- and dusk-tracking rhythms illustrate the flexibility of timing in Ipomoea nil. Molecular clock components in Arabidopsis thaliana showed complex, photoperiod-dependent regulation, which was analysed by comparison with three contrasting models. A simple, quantitative measure, Dusk Sensitivity, was introduced to compare the behaviour of clock models with varying loop complexity. Evening-expressed clock genes showed photoperiod-dependent dusk sensitivity, as predicted by the three-loop model, whereas the one- and two-loop models tracked dawn and dusk, respectively. Output genes for starch degradation achieved dusk-tracking expression through light regulation, rather than a dusk-tracking rhythm. Model analysis predicted which biochemical processes could be manipulated to extend dusk tracking. Our results reveal how an operating principle of biological regulators applies specifically to the plant circadian clock