Fitness effects of a deletion mutation increasing transcription of the 6-phosphogluconate dehydrogenase gene in Escherichia coli.

Directed evolution in microbial organisms provides an experimental approach to molecular evolution in which selective forces can be controlled and favorable mutations analyzed at the molecular level. Here we present an analysis of a mutation selected in Escherichia coli in response to growth in a chemostat in which the limiting nutrient was gluconate. The selectively favored mutation, designated gnd+ (862), occurred in the gene gnd coding for 6-phosphogluconate dehydrogenase, used in gluconate metabolism. Although the allele is strongly favored in chemostats in which the limiting nutrient is gluconate, the selective effects of gnd+ (862) are highly dependent on growth conditions. In chemostats in which growth is limited by a mixture of gluconate and either ribose, glucose, or succinate, the gnd+ (862) allele is favored, disfavored, or neutral according to the relative concentrations of the substrates. The gnd+ (862) allele results from a deletion of 385 nucleotide pairs in the region 5' to the promoter of gnd, and one endpoint of the deletion is contiguous with the terminus of an IS5 insertion sequence located near gnd in E. coli K12. The gnd+ (862) allele shows a marked increase in transcription that accounts for most or all of the increased enzyme activity.