Mutator Strains

Mutator strains introduce defined spectra of mutations at frequencies up to 100,000 times that in wild type bacteria due to defects in DNA replication and repair. The most powerful E. coli mutator strains are those containing the mutD5 allele, which enhances all types of single base-pair substitutions and single-base frameshifts. The defect is dominant and caused by two mutations within the dnaQ gene which encodes the epsilon subunit of DNA polymerase III. This catalyses the 3'->5' exonuclease (proof-reading) activity, removing mispaired nucleotides from the 3' end of the nascent DNA strand and in mutD5 strains its activity is drastically reduced. Contributing further to the mutation rate, the effectiveness of methyl-directed mismatch repair is reduced in mutD5 cells due to saturation of the mutHLS system by the excess of primary DNA replication errors.

Distribution of Mutations

mutD5 is a conditional mutator, the rate of mutation depending on growth conditions; in minimal media, cultures display a relatively low mutation rate, elevated 10-50 fold above the wild-type, but in rich media the frequency of mutations is elevated 1,000-100,000 fold. The mutation rate is independent of temperature or growth rate, and can be increased by the addition of thymidine, with maximal effect at 20 µM. All types of single base-pair substitutions and single-base frameshifts, are enhanced with a characteristic spectrum of 95% substitutions (90% transitions and 10% transversions) and 5% single-base frameshifts. Although these rate increases apply generally, the rate at which individual nucleotides mutate varies significantly and is strongly influenced by neighbouring sequences.

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