Genotype-by-environment interactions influencing the emergence of rpoS mutations in Escherichia coli populations

Polymorphisms in rpoS are common in Escherichia coli. rpoS status influences a trade-off between nutrition and stress resistance and hence fitness across different environments. To analyse the selective pressures acting on rpoS, measurement of glucose transport rates in rpoS+ and rpoS bacteria were used to estimate the role of Fnc, the fitness gain due to improved nutrient uptake, in the emergence of rpoS mutations in nutrient-limited chemostat cultures. Chemostats with set atmospheres, temperatures, pHs, antibiotics and levels of osmotic stress were followed. Fnc was reduced under anaerobiosis, high osmolarity and with chloramphenicol, consistent with a reduced rate of rpoS enrichment in these conditions. Fnc remained high however with alkaline pH and low temperature but rpoS sweeps were diminished. In these conditions, Fsp, the fitness reduction due to lowered stress protection became significant. We also estimated whether the fitness need for the gene was related to its regulation. No consistent pattern emerged between the level of RpoS and the loss of rpoS function in particular environments. This dissection allows an unprecedented view of the genotype-by-environment interactions controlling a mutational sweep, shows both Fnc and Fsp are influenced by individual stresses and that additional factors contribute to selection pressure in some environments.

Key Words: antagonistic pleiotropy;, bacterial fitness;, environmental stress;, mutational sweeps, sigma factors

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