Spontaneous mutagenesis in stressed Escherichia coli

Timms, Andrew Robert (1998). Spontaneous mutagenesis in stressed Escherichia coli. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000fec0


Mutation processes occurring in starving or stressed bacteria are different from those that are found in growing bacteria. The mutageitic response of E.coli to a number of different stress conditions, including amino acid starvation and challenge with the antibiotic streptomycin has been examined. In an E.coli trpA23 strain starved for tryptophan, the formation of small in-frame deletions was stimulated by the persistence of oxidatively damaged guanine and unrepaired mismatches in the DNA. Most (75%) of the deletions had direct repeats at each termini, one of which was lost in the deletion process. The deletions restored functionality to the product of the trpA gene enabling the mutants to form slow growing colonies on minimal plates lacking tryptophan.

During starvation of an E.coli tyrA14 strain WU3610 for the amino acid tyrosine, a new phenotypic suppressor gene (tas) was isolated that specifically complemented the absence of prephenate dehydrogenase activity. The gene is necessary for the process of starvation-associated mutagenesis in this strain; mutants do not arise during starvation in a ray deletion strain.

A high proportion (up to 20%) of ancillary mutations within the rpsL gene of E.coli have previously been found in newly arising streptomycin dependent mutants and have been shown to confer a selective advantage in established strains. This study shows that they also confer an advantage in mutants containing mixed wild type and streptomycin dependent ribosomes in the presence and absence of streptomycin. However, this selective advantage is still not enough to account for the rate at which these ancillary mutations are recovered and we need to assume a mutation rate of 10-3 to 10-4/gene/replication to explain the observed frequency.

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