The spore photoproduct lesion (SP; 5-thymine-5,6-dihydrothymine) is the dominant photoproduct found in UVirradiated spores of some bacteria such as Bacillus subtilis. Upon spore germination, this lesion is repaired in a lightindependent manner by a specific repair enzyme: the spore photoproduct lyase (SP lyase). In this work, a host-guest approach in which the N-terminal fragment of Moloney murine leukemia virus reverse transcriptase (MMLV RT) serves as the host and DNA as the guest was used to determine the crystal structures of complexes including 16 bp oligonucleotides with and without the SP lesion at 2.14 and 1.72 Å resolution, respectively. In contrast to other types of thymine-thymine lesions, the SP lesion retains normal Watson-Crick hydrogen bonding to the adenine bases of the complementary strand, with shorter hydrogen bonds than found in the structure of the undamaged DNA. However, the lesion induces structural changes in the local conformation of what is otherwise B-form DNA. The region surrounding the lesion differs significantly in helical form from B-DNA, and the minor groove is widened by almost 3 Å compared with that of the undamaged DNA. Thus, these unusual structural features associated with SP lesions may provide a basis for recognition by the SP lyase.

ABSTRACT. We aimed to evaluate the toxicity of long-term exposure to different cadmium (Cd) doses in rats and expression profiles of DNA repair-related genes. The model rats were exposed to different concentrations of CdCl 2 for 3 months, and 5 DNA repair-related genes -hMSH2, MLH1, XRCC1, hOGG1, ERCC1 -were cloned in different tissues, including the liver, kidney, heart, and lung. Accumulated amounts of Cd were detected in the tissues. Gene and protein detections were conducted via fluorescence quantitative realtime polymerase chain reaction and Western blotting, respectively. Methylated sequences of the 5 DNA repair-related gene promoters were used to investigate whether the low expression levels of the genes were related to methylation of the promoter. In the Cd-exposed group, 3 DNA repair genes (i.e., XRCC1, hOGG1, and ERCC1) significantly decreased in the rat liver, kidney, heart, and lung according to the β-actin internal standard (P < 0.01). Western blotting indicated the same trend for the different tissues. Each of the DNA repair genes had special characteristics; for example, hOGG1 gene expression decreased by 75% in the kidney, and XRCC1 gene expression decreased by 5% in the liver and heart when compared to the control group (P < 0.01). A negative correlation between the DNA repair gene expression levels and the cumulative levels of Cd was also suggested by malignancy pathology. The expression levels of 3 DNA repair genes (i.e., ERCC1, XRCC1, and hOGG1) played an important role in the rat response to Cd exposure but not DNA methylated protection.