SOS-induced DNA polymerases enhance long-term survival and evolutionary fitness

Abstract

Escherichia coli encodes three SOS-induced DNA polymerases: pol II, pol IV, and pol V. We show here that each of these polymerases confers a competitive fitness advantage during the stationary phase of the bacterial life cycle, in the absence of external DNA-damaging agents known to induce the SOS response. When grown individually, wild-type and SOS pol mutants exhibit indistinguishable temporal growth and death patterns. In contrast, when grown in competition with wild-type E. coli, mutants lacking one or more SOS polymerase suffer a severe reduction in fitness. These mutants also fail to express the "growth advantage in stationary phase" phenotype as do wild-type strains, instead expressing two additional new types of "growth advantage in stationary phase" phenotype. These polymerases contribute to survival by providing essential functions to ensure replication of the chromosome and by generating genetic diversity.

Figure 1

Long-term survival patterns of wild-type and polymerase mutant cells. (A) Composite of survival patterns when cells are grown alone in batch culture. (B) Composite of survival patterns of each pol⁻ strain when grown in competition with wild-type cells. Representative growth curves are shown. Solid line with filled boxes, wild-type; dashed lines are mutant strains: open squares, pol II⁻; open circles, pol IV⁻; open triangles, pol V⁻ mutants. CFU/ml, colony-forming units per milliliter. Asterisks correspond to cell titers below the limit of detection of 100 CFU/ml.

Figure 2

Transcription of the SOS-induced DNA polymerases. RT-PCR assays were performed on wild-type (WT), polB (pol II⁻), dinB (pol IV⁻), or umuDC (pol V⁻) cells harvested from 1-day-old cultures (A) or 5-day-old wild-type cultures (B). Gene names are indicated above each lane. The expected sizes of RT-PCR products are: sbmC, 311 bp; recA, 328 bp; umuD, 298 bp; dinB 206 bp, polB, 367 bp; dps, 209 bp.

Figure 3

Representative competitions between polymerase-deficient strains. Solid lines with open squares, pol IV⁻; dashed lines with open circles, pol V⁻. CFU/ml, colony forming units per milliliter.

Figure 4

GASP competition phenotypes of wild-type and polymerase mutant strains during coculture. Competition phenotypes are defined as: class 1, strong GASP phenotype; class 2, weaker GASP phenotype; class 3, abortive expression of GASP; class 4, no expression of the GASP phenotype. Representative examples of each class are shown. Black lines represent titers of unaged populations; gray lines represent titers of cells obtained from 10-day-old aged cultures. The frequency, in percentage, of expression of each class of GASP competition phenotype for wild-type and mutant strains is shown. n = 14 for wild-type (WT), pol II⁻, and pol IV⁻ strains; n = 18 for pol V⁻ strains. CFU/ml, colony forming units per milliliter.