Copyright zyxwvutsrqponm 0 1988 by the Genetics Society of America DNA Repair and the Evolution of Transformation in the Bacterium zyxw Bacillus subtilis Richard E. Michod, Martin F. Wojciechowski and Mary A. Hoelzer Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona zyx 85721 zyx Manuscript received June zyxwv 12, 1987 Revised copy accepted October 6, 1987 ABSTRACT The purpose of the work reported here is to test the hypothesis that natural genetic transformation in the bacterium Bacillus subtilis has evolved asa DNA repair system. Specifically, tests were made to determine whether transformation functions to provide DNA template for the bacterialcell to use in recombinational repair. The survivorship and the homologous transformation rate as a function of dose of ultraviolet irradiation (UV) was studied in two experimental treatments, in which cells were either transformed before (DNA-UV), or after (UV-DNA), treatment with UV. The results show that there is a qualitative difference in the relationship betweenthe survival of transformed cells (sexual cells) and total cells (primarily asexual cells) in the two treatments. As predicted by the repair hypothesis, in the UV-DNA treatment, transformed cells had greater average survivorship than total cells, while in the DNA-UV treatment this relationship was reversed. There was also a consistent and qualitative difference between the UV-DNA and DNA-UV treatments in the relationship between the homologous transformation rate (transformed cells/total cells) and UV dosage. As predicted by the repair hypothesis, the homologous transformation rate increases with UV dose in the UV-DNA experiments but decreases with UV dose in the DNA-UV treatments. However, the transformation rate for plasmid DNA does not increase in a UV-DNA treatment. These results support the DNA repair hypothesis for the evolution of transformation in particular, and sex generally. S EXUAL systems in both eukaryotes and prokar- yotes involve two basic components, (1) recom- bination, in the sense of the breakage and reunionof two homologous DNA molecules, and (2) outcrossing, in the sense that the two homologous DNAmolecules involved in recombination come from two different individuals (for discussion, see MICHOD and LEVIN 1987). Both recombination and outcrossing occur in natural transformation, a process in which some bac- teria take up, and recombine into their own genome, DNA released from other bacteria or provided exper- imentally. The purpose of the work reported here is to test the DNA repair hypothesis (BERNSTEIN, BYERS and MICHOD 1981 zyxwvutsrq ; BERNSTEIN et al. 1984,1985a-c; BERN- STEIN, HOPF and MICHOD 1987a, b) as an explanation for the evolution of naturaltransformation in the eubacterium Bacillus subtilis. Because many of the molecular and cellular details of transformation (for review, see DUBNAU 1982) and DNA repair (for re- view, see YASBIN 1985) have been elucidated, B. sub- tilis provides an ideal organism to experimentally ad- dress predictions of the repair hypothesis. From what is known about the mechanisms involved, natural transformation in B.subtilis appears to be a highly evolved trait. It results from a complex, energy-re- quiring, developmental process and notpassive entry of DNA into the cell or artificial manipulation of the Genetics 118: 31-39 zyxwvutsrqp (January, 1988) cell. Its genetic control involves genes involved in both homologous recombination and DNA repair. However, although much is known about the mecha- nisms involved, the evolutionary function of natural transformation is poorly understood. The repair hypothesis argues that the evolutionary function of transformation lies in its role in providing DNA template for recombinational repair of genetic damages. T o study this hypothesis, we have measured the densities of transformed cells and total cells at different DNA concentrations in the presence of in- creasing levels of UV radiation. Under common lab- oratory conditions, a culture of B. subtilis grown to competence is a mixture of predominately noncom- petent (asexual) cells and a minority of approximately 10-20% competent (sexual) cells (SOMMA and POLSI- NELLI 1970; DUBNAU 1982). Competence refers to the physiological state of the cell in which it can be trans- formed, that is, in which itcanbind, take up and recombine free DNA which may have been released from another cell or provided experimentally. Be- cause the total population of a competent culture is predominatelycomposedofasexual cells, we view comparisons of transformed cells with total cells as indicative of comparisons of sexual and asexual cells. However, because the population of total cells is a mixture of asexual and sexual cells, the differences we observe between the transformed cells and the total