The overwhelming majority of DNA photoproducts in UVirradiated spores is a unique thymine dimer called spore photoproduct (SP, 5-thymine-5,6-dihydrothymine). This lesion is repaired by the spore photoproduct lyase (SP lyase) enzyme that directly reverts SP to two unmodified thymines. The SP lyase is an S-adenosylmethionine-dependent iron-sulfur protein that belongs to the radical S-adenosylmethionine superfamily. In this study, by using a well characterized preparation of the SP lyase enzyme from Bacillus subtilis, we show that SP in the form of a dinucleoside monophosphate (spore photoproduct of thymidilyl-(3-5)-thymidine) is efficiently repaired, allowing a kinetic characterization of the enzyme. The preparation of this new substrate is described, and its identity is confirmed by mass spectrometry and comparison with authentic spore photoproduct. The fact that the spore photoproduct of thymidilyl-(3-5)-thymidine dimer is repaired by SP lyase may indicate that the SP lesion does not absolutely need to be contained within a singleor double-stranded DNA for recognition and repaired by the SP lyase enzyme.The DNA of all organisms is subject to modifications upon exposure to a wide variety of chemical and physical agents. Among them, solar ultraviolet radiation is known to induce dimerization reactions between adjacent pyrimidines (1). In the vast majority of living systems, the resulting photoproducts are cyclobutane pyrimidine dimers (CPDs) 5 and pyrimidine (6-4) pyrimidone photoproducts (Scheme 1A) that can be generated at any of the four bipyrimidine doublets (TT, CT, TC and CC), although the yields of the lesions depend on the bases involved (2). These lesions induce mutations and can be lethal because of blocking of the replication machinery. The photochemistry in bacterial spores is quite different. Indeed, in this dormant form produced by some bacteria such as Bacillus subtilis, the only photoproduct produced upon exposure to UV light corresponds to two thymines linked by the methyl group of one of the bases (3, 4). The formation of this specific lesion, 5-thyminyl-5,6-dihydrothymine (spore photoproduct, SP) (Scheme 1A), is explained by specific features of the spores, including DNA conformation (A form), dehydration, the presence of dipicolinic acid in the core, and binding of small acid-soluble proteins to DNA (5-8). The formation of SP as the unique DNA lesion in irradiated spores is proposed to account for their extreme resistance to UV radiation. Indeed, spores express a specific repair enzyme, the spore photoproduct lyase (SP lyase) that directly reverts SP to two unmodified thymines upon germination (9, 10), much more efficiently than dimeric photoproducts are removed from other cell types by the classical nucleotide excision repair pathway. The specific photochemistry of DNA in spores combined with the action of SP lyase appears to be a major evolutionary advantage for spore-forming bacteria in resistance to UV radiation.In their N-terminal half, all SP lyase enzymes contain a strictly conserved a...

By irradiation of bacterial spores under UV radiation, a photoproduct (SP) bearing a covalent methylene link between two adjacent thymines is formed in DNA. Because of the presence of an asymmetric carbon on the aglycone and of two possible orientations for the formation of the cross-link, four isomers could in principle be obtained. Currently, no conclusive structural information of this photoproduct is available. The structure of the isolated SPTpT dinucleotide was revisited in order to determine the type of cross-link and the absolute configuration of the C5a carbon. For this purpose, a study combining NMR spectroscopy and DFT calculations was pursued on the spore photoproduct of the dinucleoside TpT since its structure was previously shown to be identical to the one produced in DNA. A full characterization of SPTpT by NMR analyses was performed in D2O and DMSO. 2D NMR measurements (1H-13C, 1H-31P, COSY, NOESY, and ROESY) and DFT calculations (geometries optimization of R and S isomers and theoretical chemical shifts) lead us to conclude without ambiguity that the absolute configuration of the C5a carbon is R and that the methylene bridge of the photoproduct corresponds to the methyl group of the thymine located on the 3'-end of the dinucleoside monophosphate.