Parallel genetic and phenotypic evolution of DNA superhelicity in experimental populations of Escherichia coli

DNA supercoiling is the master function that interconnects chromosome structure and global gene transcription. This function has recently been shown to be under strong selection in Escherichia coli. During the evolution of 12 initially identical populations propagated in a defined environment for 20,000 generations, parallel increases in DNA supercoiling were observed in ten populations. The genetic changes associated with the increased supercoiling were examined in one population, and beneficial mutations in the genes topA (encoding topoisomerase I) and fis (encoding a histone-like protein) were identified. To elucidate the molecular basis and impact of these changes, we quantified the level of genetic, phenotypic, and molecular parallelism linked to DNA supercoiling in all 12 evolving populations. First, sequence determination of DNA topology-related loci revealed strong genetic parallelism, with mutations concentrated in three genes (topA, fis, and dusB), although the populations had different alleles at each locus. Statistical analyses of these polymorphisms implied the action of positive selection and, moreover, suggested that fis and dusB, which belong to the same operon, have related functions. Indeed, we demonstrated that DusB regulates the expression of fis by both experimental and phylogenetic analyses. Second, molecular analyses of five mutations in fis and dusB affecting the transcription, translation, and protein activity of Fis also revealed strong parallelism in the resulting phenotypic effects. Third, artificially increasing DNA supercoiling in one of the two populations that lacked DNA topology changes led to a significant fitness increase. The high levels of molecular and genetic parallelism, targeting a small subset of the many genes involved in DNA supercoiling, indicate that changes in DNA superhelicity have been important in the evolution of these populations. Surprisingly, however, most of the evolved alleles we tested had either no detectable or slightly deleterious effects on fitness, despite these signatures of positive selection.