DNA-REPAIR AND THE EVOLUTION OF TRANSFORMATION .4. DNA-DAMAGE INCREASES TRANSFORMATION

Natural genetic transformation in the bacterium Bacillus subtilis prov ides a model system to explore the evolutionary function of sexual rec ombination. In the present work, we study the response of transformati on to UV irradiation using donor DNAs that differ in sequence homology to the recipient's chromosome and in the mechanism of transformation. The four donor DNAs used include homologous-chromosomal-DNA, two plas mids containing a fragment of B. subtilis trp+ operon DNA and a plasmi d with no sequence homology to the recipient cell's DNA. Transformatio n frequencies for these DNA molecules increase with increasing levels of DNA damage (UV radiation) to recipient cells, only if their transfo rmation requires homologous recombination (i.e. is recA+-dependent). T ransformation with non-homologous DNA is independent of the recipient' s recombination system and transformation frequencies for it do not re spond to increases in UV radiation. The transformation frequency for a selectable marker increases in response to DNA damage more dramatical ly when the locus is present on small, plasmid-borne, homologous fragm ents than if it is carried on high molecular weight chromosomal fragme nts. We also study the kinetics of transformation for the different do nor DNAs. Different kinetics are observed for homologous transformatio n depending on whether the homologous locus is carried on a plasmid or on chromosomal fragments. Chromosomal DNA- and non-homologous-plasmid -DNA-mediated transformation is complete (maximal) within several minu tes, while transformation with a plasmid containing homologous DNA is still occurring after an hour. The results indicate that DNA damage di rectly increases rates of homologous recombination and transformation in B. subtilis. The relevance of these results and recent results of o ther labs to the evolution of transformation are discussed.