While the distribution of the various photoproducts induced by UVB and UVC radiation is the same in the isolated DNA of most cell types, it is totally different in bacterial spores. These are dormant forms of certain bacteria induced by stress conditions such as lack of nutrients, lack of humidity or exposure to certain radiation. Spores are much more resistant than vegetative bacteria, particularly to UVB and UVC radiation. This is partly due to the formation of a particular dimeric photoproduct, consisting of a bridge between two thymines via one of the methyl groups. This lesion, 5-(α-thyminyl)-5,6-dihydrothymine or "spore photoproduct" (SP), is the only one found in Bacillus subtilis spores exposed to UVC radiation. This unique photochemistry is partly explained by spore dehydration. However, it is above all the presence of large quantities of DNA-bound proteins(small, acid soluble proteins, SASP) and dipicolinic acid (DPA) in the spore that induces this specificity. The link between spore photoresistance and SP formation is undoubtedly the presence of a highly efficient repair enzyme, spore photoproduct lyase (SPL), which breaks the link between the two thymines at germination. SPL is a metalloenzyme belonging to the large family of "radical-SAM" proteins, which uses a particular [4Fe-4s] center and S-adenosylmethionine to produce the 5'-deoxyadenosyl radical responsible for SP repair.
In the course of the seminar, we will first explain the factors that determine the formation of the spore photoproduct and its mechanism of formation, and then explain why and how this lesion is rapidly repaired at the very start of germination by SPL.
