The main lesion produced in DNA by UV-C irradiation of spores of Bacillus subtilis is 5-thyminyl-5,6-dihydrothymine (spore photoproduct [SP]). In contrast, cyclobutane pyrimidine dimers (CPD) and pyrimidine (6-4) pyrimidone photoproducts (6-4PP) are the main photolesions in other cell types. The novel photochemistry of spore DNA is accounted for in part by its reduced hydration, but largely by the saturation of spore DNA with α/β-type small, acid-soluble spore proteins (SASP). Using high-performance liquid chromatography–mass spectrometry analysis of the photoproducts, we showed that in wild-type B. subtilis spores (1) UV-C irradiation generates almost exclusively SP with little if any CPD and 6-4PP; (2) the SP generated is ∼99% of the intrastrand derivative, but ∼1% is in the interstrand form; and (3) there is no detectable formation of the SP analog between adjacent C and T residues. UV-C irradiation of spores lacking the majority of their α/β-type SASP gave less SP than with wild-type spores and significant levels of CPD and 6-4PP. The binding of an α/β-type SASP to isolated DNA either in dry films or in aqueous solution led to a large decrease in the yield of CPD and 6-4PP, and a concomitant increase in the yield of SP, although levels of interstrand photoproducts were extremely low.
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Vol. 81 • No. 1