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Cells are constantly exposed to Reactive Oxygen Species (ROS) produced both endogenously to meet physiological requirements and from exogenous sources. While endogenous ROS are considered as important signalling molecules, high uncontrollable ROS are detrimental. It is unclear how cells can achieve a balance between maintaining physiological redox homeostasis and robustly activate the antioxidant system to remove exogenous ROS. We have utilised a Systems Biology approach to understand how this robust adaptive system fulfils homeostatic requirements of maintaining steady-state ROS and growth rate, while undergoing rapid readjustment under challenged conditions. Using a panel of human ovarian and normal cell lines, we experimentally quantified and established interrelationships between key elements of ROS homeostasis. The basal levels of NRF2 and KEAP1 were cell line specific and maintained in tight correlation with their growth rates and ROS. Furthermore, perturbation of this balance triggered cell specific kinetics of NRF2 nuclear-cytoplasmic relocalisation and sequestration of exogenous ROS. Our experimental data were employed to parameterise a mathematical model of the NRF2 pathway that elucidated key response mechanisms of redox regulation and showed that the dynamics of NRF2-H2O2 regulation defines a relationship between half-life, total and nuclear NRF2 level and endogenous H2O2 that is cell line specific.

Experimental evolution studies are used to investigate bacterial adaptive radiation in simple microcosms. In the case of the Wrinkly Spreader, a class of biofilm-forming adaptive mutants of Pseudomonas fluorescens SBW25, the current paradigm is that they are only evolutionarily successful in static microcosms where they outcompete other lineages for O2 at the air-liquid interface. However, we have isolated Wrinkly Spreaders from drip-fed glass bead columns as an example of parallel evolution. These mutants are adaptive, with competitive fitness advantages on columns of 1.28-1.78. This might be explained by the enhanced attachment characteristically shown by Wrinkly Spreaders, allowing them to resist liquid flow through the column pore network. A comparison of column and static microcosm-isolated Wrinkly Spreaders showed that many aspects of wrinkleality, including colony reversion, microcosm growth, biofilm strength and attachment, as well as fitness in static microcosms, were significantly different within and between the two groups of mutants. These findings indicate that the two environments had selected for Wrinkly Spreaders with subtly differing degrees of wrinkleality and fitnesses, suggesting that aspects of the Wrinkly Spreader phenotype may have different relative values in static microcosms and drip-fed columns.

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