Time-dependent accumulation of mutants in several genetic systems has led to the controversial conclusion that bacteria possess a mechanism to elevate general mutagenesis in response to stress. In particular, extensive study of the Cairns system has kept the controversy alive despite theoretical analysis suggesting that general mutagenesis is counter-productive. In the Cairns system, a lac mutant strain gives rise to Lac+ revertant colonies that accumulate above a non-growing lawn over a week. Two conflicting models explain this behavior. The Stress-Induced Mutagenesis model proposes that cells have a mechanism that creates genome-wide mutations during growth limitation that may relieve the stress. Most mutations are attributed to action of the error-prone Pol IV polymerase during recombination-dependent double-stranded break repair in non-growing cells. We have directly tested this model by measuring mutation rates during non-selective growth (the only condition under which a true mutation rate can be measured). Cell functions that are proposed to change in response to stress (SOS, Pol IV and recombination) and cause mutagenesis are given or removed from growing cultures. We find that none of these factors affect mutation rate in growing cells. In addition, we find that vast (non-physiological) over-expression of Pol IV causes mutagenesis that shows no dependence on recombination. Results are conflicting with predictions of the Stress-Induced Mutagenesis model.
Results are explained by the alternative Growth-Under-Selection model in which pre-existing small-effect mutations initiate slow-growing clones under selection. Within developing colonies, secondary genetic improvements occur without enhanced mutagenesis. These mutations are made more likely by the increasing number of potential targets for mutations within a growing colony. We provide evidence that revertant colonies appearing under selection are initiated by pre-existing cells with an amplification of the mutant lac gene. By killing these pre-existing cells, we prevent appearance of revertant colonies on selective plates. This demonstrates that revertant number is determined before selection cannot be created in response to stress.
|Advisor:||Roth, John R.|
|Commitee:||Hunter, Neil, Syvanen, Michael|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-B 73/07(E), Dissertation Abstracts International|
|Subjects:||Genetics, Microbiology, Evolution and Development|
|Keywords:||Bacteria, Genes, Rapid adaptation, Selection|
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