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mut-25, a mutation to mutator linked to purA in Escherichia coli
Abstract:
The mutation mut-25 that results in a mutator phenotype is closely linked to purA on the chromosome of Escherichia coli. The gene order in this region is ampA mut-25 purA. purA mut-25 double mutants retained mutator activity indicating that mut-25 is not a mutation in the purA gene. The repair mutations uvrA6, recA56, and exrA1 had no effect on mutation frequencies in mut-25 strains, and mut-25 strains were normally resistant to ultraviolet irradiation. Frequencies of host range mutations were not increased in phages T1, T2, and T7 grown on mut-25 strains. mut-25 could act trans, reverting the trpA46 mutation either on the chromosome or on an F episome. The transitions AT yields GC (adenine-thymine yields guanine-cytosine) and GC yields AT were induced by mut-25.
Insights
The Escherichia coli mut-25 mutation causes a mutator phenotype, increasing mutation rates. This mutation specifically induces AT to GC and GC to AT transitions, independent of DNA repair pathways.
Area of Science:
- Microbiology and Molecular Genetics
- Bacterial Genetics and Mutagenesis
Background:
- The mut-25 mutation in Escherichia coli confers a mutator phenotype, characterized by an elevated mutation rate.
- The genetic locus of mut-25 is closely associated with the purA gene on the E. coli chromosome, with the gene order ampA mut-25 purA.
Purpose of the Study:
- To investigate the nature of the mut-25 mutation and its impact on mutation frequencies and DNA repair.
- To determine the specific types of mutations induced by mut-25 and its genetic behavior.
Main Methods:
- Genetic mapping of mut-25 relative to purA.
- Analysis of mutation frequencies in double mutants with known repair genes (uvrA6, recA56, exrA1).
- Assessment of ultraviolet (UV) irradiation resistance.
- Evaluation of host range mutations in bacteriophages T1, T2, and T7.
- Complementation tests (trans-acting ability) and reversion assays of trpA46 mutation.
Main Results:
- mut-25 is distinct from purA, as indicated by retained mutator activity in purA mut-25 double mutants.
- DNA repair mutations (uvrA6, recA56, exrA1) did not affect mutation frequencies in mut-25 strains, which showed normal UV resistance.
- mut-25 did not increase host range mutations in tested phages and exhibited trans-acting capability.
- The mut-25 mutation specifically induced AT to GC and GC to AT base-pair transitions.
Conclusions:
- The mut-25 mutation is a distinct genetic element responsible for a mutator phenotype in Escherichia coli.
- This mutator activity is independent of major DNA repair pathways (UvrA, RecA, ExrA) and does not affect UV sensitivity.
- mut-25 specifically promotes transition mutations (AT <-> GC) and can function in a trans-acting manner.