title: Evidence for Retromutagenesis as a Mechanism for Adaptive Mutation in Escherichia coli.
abstract: Adaptive mutation refers to the continuous outgrowth of new mutants from a non-dividing cell population during selection, in apparent violation of the neo-Darwinian principle that mutation precedes selection. One explanation is that of retromutagenesis, in which a DNA lesion causes a transcriptional mutation that yields a mutant protein, allowing escape from selection. This enables a round of DNA replication that establishes heritability. Because the model requires that gene expression precedes DNA replication, it predicts that during selection, new mutants will arise from damage only to the transcribed DNA strand. As a test, we used a lacZ amber mutant of Escherichia coli that can revert by nitrous acid-induced deamination of adenine residues on either strand of the TAG stop codon, each causing different DNA mutations. When stationary-phase, mutagenized cells were grown in rich broth before being plated on lactose-selective media, only non-transcribed strand mutations appeared in the revertants. This result was consistent with the known high sensitivity to deamination of the single-stranded DNA in a transcription bubble, and it provided an important control because it demonstrated that the genetic system we would use to detect transcribed-strand mutations could also detect a bias toward the non-transcribed strand. When residual lacZ transcription was blocked beforehand by catabolite repression, both strands were mutated about equally, but if revertants were selected immediately after nitrous acid exposure, transcribed-strand mutations predominated among the revertants, implicating retromutagenesis as the mechanism. This result was not affected by gene orientation. Retromutagenesis is apt to be a universal method of evolutionary adaptation, which enables the emergence of new mutants from mutations acquired during counterselection rather than beforehand, and it may have roles in processes as diverse as the development of antibiotic resistance and neoplasia.
authors: Department of Biochemistry, Emory University School of Medicine, Atlanta, Georgia, United States of America; Graduate Program in Genetics and Molecular Biology, Emory University School of Medicine, Atlanta, Georgia, United States of America.
authors: Jordan
authors: J
authors: Morreall
authors: Department of Biochemistry, Emory University School of Medicine, Atlanta, Georgia, United States of America.
authors: Alice
authors: A
authors: Kim
authors: Department of Biostatistics and Bioinformatics, Rollins School of Public Health, Emory University School of Medicine, Atlanta, Georgia, United States of America.
authors: Yuan
authors: Y
authors: Liu
authors: Department of Biochemistry, Emory University School of Medicine, Atlanta, Georgia, United States of America.
authors: Natalya
authors: N
authors: Degtyareva
authors: Department of Pathology, Emory University School of Medicine, Atlanta, Georgia, United States of America.
authors: Bernard
authors: B
authors: Weiss
authors: Department of Biochemistry, Emory University School of Medicine, Atlanta, Georgia, United States of America; Emory Winship Cancer Institute, Emory University School of Medicine, Atlanta, Georgia, United States of America; Department of Radiation Oncology, Emory University of School of Medicine, Atlanta, Georgia, United States of America; Hematology and Medical Oncology, Emory University School of Medicine, Atlanta, Georgia, United States of America.
authors: Paul W
authors: PW
authors: Doetsch
mesh_terms: D000220:Adaptation, Biological; D004926:Escherichia coli / Q000235:genetics*; D019143:Evolution, Molecular; D005798:Genes, Bacterial; D008957:Models, Genetic; D016296:Mutagenesis