The role of recombinational repair in the evolution of natural transformation in Bacillus subtilis.
AuthorHoelzer, Mary Ann.
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PublisherThe University of Arizona.
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AbstractThe repair hypothesis argues that natural genetic transformation in Bacillus subtilis evolved as a system for recombinational repair of damages in the recipient cell's genome. Cell survivorship and the apparent transformation frequency were examined in a rec+ strain as well as in two repair-deficient mutant strains. One of the mutant strains YB1005 was defective in its ability to carry out excision repair, the second mutant strain YB1260 was deficient in its ability to elicit an SOS-like response to DNA damage or following the onset of competence. Cells were subjected to two experimental treatments that differ in the order of administration of damage and transforming donor DNA. In one treatment cells were exposed to UV radiation or damage first and then allowed to undergo recombination with transforming donor DNA (UV-DNA). In the second treatment the cells were transformed first and then subjected to UV radiation (DNA-UV). In order to more closely simulate the effects of transformation in natural populations the YB886 rec+ strain was transformed using damaged and undamaged donor DNA. In nature donor DNA is likely to come from neighboring cells exposed to similar levels of damage. Experimental results show a qualitative difference in the relationship between the survival of transformed and total cells in the two treatments. The density of competent cells increases, relative to noncompetent cells with increasing UV dose, when cells are transformed after damage. There was also a consistent difference between the UV-DNA and DNA-UV treatments in the relationship between the apparent transformation frequency and UV dose. The apparent transformation frequency increases with increasing UV dose when the cells are damaged prior to transformation. The results persist in the two mutant strains examined, therefore the greater increase in the apparent transformation frequency does not depend upon excision repair or inducible SOS-like repair. In addition, the apparent benefit of transformation remains even when the donor DNA is derived from damage cells. All of the results reported here are consistent with the repair hypothesis for the evolution of natural transformation in Bacillus subtilis. They provide empirical support for the hypothesis that the evolutionary function of competence is to bring DNA into the cell for use as template in recombinational repair of DNA damage.
Degree ProgramEcology and Evolutionary Biology