Evidence for Retromutagenesis as a Mechanism for Adaptive Mutation in Escherichia coli.

Jordan Morreall,Alice Kim,Natalya Degtyareva,Bernard Weiss,Yuan Liu,Paul W Doetsch,William F Burkholder

doi:10.1371/journal.pgen.1005477

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.

Highlights

A central tenet of genetics is that mutation precedes selection rather than the reverse
The basic principle of neo-Darwinian genetics is that mutations occurring during growth enable the subsequent survival of the mutants under selective environmental conditions
New mutants can arise from a non-growing bacterial population during selection in an apparently Lamarckian way

Summary

Introduction

A central tenet of genetics is that mutation precedes selection rather than the reverse. This principle was verified by isolation, through sibling selection, of mutants from cultures that had never been exposed to the growth pressure of a selective agent [1,2]. In apparent contradiction to this principle, it was later observed that new mutants would arise continuously from an ostensibly non-growing population of cells for many days after being plated on a selective medium [3,4,5]. Controversy still exists around the contributions of residual growth and enhanced mutation rates, and so adaptive mutagenesis is best defined as the process by which new mutations arise under selective conditions regardless of the mechanism [7]

Methods

Results

Discussion

Conclusion