Gene dosage toxicity (GDT) is an important factor that determines optimal levels of expression of many genes and tradeoffs associated with escape of bacterial species from stressors such as antibiotics. We studied the mechanistic origin of GDT caused by overexpression of an essential E. coli enzyme Dihydrofolate Reductase (DHFR). We found that overexpression of DHFR causes major metabolomic shifts in E. coli which can be partly compensated by the variation of media which partially rescues the growth defect caused GDT. Surprisingly, DHFR enzyme from several ‘evolutionarily distant cousins’ of E. coli were not toxic when over-expressed in E. coli. We found that GDT is caused by excessive protein-protein interactions between DHFR and metabolically related enzymnes, most of which use DHFR product, tetrahydrofolate as the source of one carbon metabolism. In vitro studies revealed that presence of enzymatic neighbors enhances activity of the enzymes in a highly non-linear (with concentration) manner suggesting strong allosteric effects. Quite remarkably such allosteric enhancement is observed only in mixtures of proteins from the same bacterial species. When orthologious proteins from distant bacterial species were mixed, the effect of functional enhancement were not observable and in some cases we observed functional suppression. Altogether these data point out to a new phenomenon of collective allosteric interactions and its evolutionary significance. We will discuss possible mechanistic explanations of these observations.

Allosteric effects of weak transient protein-protein interactions: Crowding in cells and evolutionary consequences

253rd National Meeting (2017)

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