Physiological characterization of adaptive evolutionary dynamics

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Synopsis

In bacteria, gene expression follows simply from the central dogma, DNA-to-RNA-protein, with each step modulated by regulatory molecules. Yet the synthesis rate of each of these macromolecules, and the abundance of their precursors, exhibits strong growth rate dependence imposing intrinsic constraints on gene expression. One such constraint in the bacterium Escherichia coli is that the total protein concentration remains approximately growth-rate-independent and so proteome allocation shapes gene expression: If, for example, a co-regulated sector of the proteome increases in concentration, other protein concentrations must decrease to maintain a constant overall concentration. Proteome allocation constraints manifest as simple ‘growth laws’ that link the growth state (consisting of the doubling rate, ribosome protein abundance and other reporters of the physiological state) to the macromolecular composition of the cell. These growth laws are an intermediate measure between genotype and phenotype that can provide insight into the dynamics of adaptive long-term laboratory evolution. Applied to Lenski's strains, which have been adapted to growth on glucose minimal media for tens of thousands of generations, we can connect changes in proteome composition to large-scale changes in the cell physiology.