Originally published as Genetics Published Articles Ahead of Print on February 19, 2006.

Genetics, Vol. 172, 2071-2079, April 2006, Copyright © 2006
doi:10.1534/genetics.105.053892

Genotype-by-Environment Interactions Influencing the Emergence of rpoS Mutations in Escherichia coli Populations

Thea King, Shona Seeto and Thomas Ferenci¹

School of Molecular and Microbial Biosciences, University of Sydney, Sydney, New South Wales 2006, Australia

¹ Corresponding author: School of Molecular and Microbial Biosciences, G08, University of Sydney, Sydney, NSW 2006, Australia.
E-mail: tferenci{at}mail.usyd.edu.au

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 analyze the selective pressures acting on rpoS, measurement of glucose transport rates in rpoS⁺ and rpoS bacteria was used to estimate the role of F_nc, the fitness gain due to improved nutrient uptake, in the emergence of rpoS mutations in nutrient-limited chemostat cultures. Chemostats with set atmospheres, temperatures, pH's, antibiotics, and levels of osmotic stress were followed. F_nc was reduced under anaerobiosis, high osmolarity, and with chloramphenicol, consistent with a reduced rate of rpoS enrichment in these conditions. F_nc remained high, however, with alkaline pH and low temperature but rpoS sweeps were diminished. Under these conditions, F_sp, 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 and shows that both F_nc and F_sp are influenced by individual stresses and that additional factors contribute to selection pressure in some environments.

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