Stress-Induced Mutagenesis

Abstract

Early research on the origins and mechanisms of mutation led to the establishment of the dogma that, in the absence of external forces, spontaneous mutation rates are constant. However, recent results from a variety of experimental systems suggest that mutation rates can increase in response to selective pressures. This chapter summarizes data demonstrating that,under stressful conditions, Escherichia coli and Salmonella can increase the likelihood of beneficial mutations by modulating their potential for genetic change.Several experimental systems used to study stress-induced mutagenesis are discussed, with special emphasison the Foster-Cairns system for "adaptive mutation" in E. coli and Salmonella. Examples from other model systems are given to illustrate that stress-induced mutagenesis is a natural and general phenomenon that is not confined to enteric bacteria. Finally, some of the controversy in the field of stress-induced mutagenesis is summarized and discussed, and a perspective on the current state of the field is provided.

Figure 1

Adaptive mutation to Lac⁺ in E. coli strain FC40. Six cultures of FC40 were grown to saturation in liquid M9-glycerol medium. Aliquots containing 3 X 10⁸ FC40 cells were mixed with 10⁹ scavenger cells and spread on M9-lactose plates. On each day, small circular samples were removed from one of each set of six plates (avoiding any visible Lac⁺ colonies) and were vortexed with 1 ml M9; the viable titer of FC40 in these suspensions was assayed on rifampicin-peptone plates (filled circles). The Lac⁺ colony counts (open circles) are the averages for 23 plates. (Reproduced with permission from The Genetics Society of America [43].)

Figure 2

The genetic structure of E. coli strain FC40. FC40 is deleted for the (lac-pro) region of the chromosome. A region of chromosomal DNA on the F′128 episome, which includes the Φ(lacI33-lacZ) allele, complements this deletion. The Φ(lacI33-lacZ) allele is a fusion of lacI to lacZ and is expressed from the constitutive lacI^q promoter. This fusion normally encodes a functional β-galactosidase protein; however, FC40 is Lac^- due to insertion of an extra guanine residue in the lacI region of the fusion. Adaptive Lac⁺ reversions occur when a -1 frameshift restores the normal reading frame in the Φ(lacI33-lacZ) allele (see text for additional details).

Figure 3

The recombination-dependent model for adaptive mutation to Lac⁺. A replication fork initiated at the vegetative origin, oriS, on F′128 collapses when it arrives at a nick at the conjugal origin, oriT. A. Collapse of the replication fork creates a double-strand end. B. RecBCD processes the double-strand end to form a 3′ single-strand end. C. RecA catalyzes the invasion of the 3′ single-strand end into a homologous region of duplex DNA. D. PriA-dependent DNA synthesis is initiated from the invading 3′′end by DNA Pol IV or Pol II and a Holliday junction is formed. E. A normal replication fork is re-established with DNA Pol III. The Holliday junction is processed and resolved by RuvABC. Adaptive Lac⁺ reversions occur when error-prone DNA synthesis extends into the lac region on the episome and introduces a -1 frameshift.

Figure 4

The amplification-dependent model for adaptive mutation to Lac⁺. The mutant lac allele on F′128 is spontaneously duplicated. During incubation on lactose, selection for increased β-galactosidase activity favors further amplification of the lac region. A true Lac⁺ reversion occurs in a copy of the mutant lac allele in the amplified array. Once a true Lac⁺ allele is present, the selective pressure promoting lac amplification is relieved and the lac copy number decreases. Finally, a stable Lac⁺ revertant cell with a single copy of lac is formed and this cell grows to form a colony on the minimal lactose plate. (Adapted from Annual Reviews in Microbiology [210] with permission from the publisher.)