Fig 6. Illustration of investment in toxin removal while maximising growth rate.
(A) The example network contains five metabolites; x1 to x4 are precursor molecules and x3 and x4 play the role of amino acids. An external toxin can diffuse over the membrane and, when inside the cell, inhibits the catalytic activity of all proteins. A fifth protein, in purple, destroys the toxin. (B) Without toxin present, investment is divided over the four proteins and the ribosome to maximise steady state growth rate. With toxin present, the toxin-degrading protein is also expressed, even though it does not directly contribute to growth. However, without expressing this stress protein, the maximal growth rate would have been lower. (C) Investment in proteins and ribosomes to attain maximal balanced growth rate as a function of toxin concentration. Note that in this example, a switch is observed, from an EGM with four metabolites to one with five. Full detail and code may be found in S1 Text.