UV radiation of DNA generates three major types of pyrimidine dimers, cyclobutane pyrimidine dimers (CPDs), pyrimidine (6-4) pyrimidone photoproducts (6-4PPs) and 5-thyminyl-5,6-dihydrothymine, i.e. the spore photoproduct (SP). All these dimers can be repaired by direct reversal enzymes where CPDs and 6-4PPs are repaired by the respective photolyase and SP is repaired by spore photoproduct lyase (SPL). The CPD photolyase was shown to exhibit ~ 100,000-fold higher affinity toward CPD in CPD-containing DNA than undamaged DNA, explaining the highly efficient CPD repair in duplex DNA by the enzyme. It was widely expected that SPL may exhibit a similarly high affinity toward SP in duplex DNA leading to an efficient SP repair. Surprisingly, our latest studies using SP-containing short oligonucleotides, pUC 18 plasmid DNA, and E. coli genomic DNA found that they are all poor substrates for SPL in general, exhibiting turnover numbers of 0.01-0.2 min-1. The faster turnover numbers are reached under single turnover conditions, and SPL activity is lower with oligonucleotide substrates at higher concentrations. Moreover, SP-containing oligonucleotides do not go past one turnover. In contrast, the dinucleotide SP TpT exhibits a turnover number of 0.3 ~ 0.4 min-1, and the reaction may reach up to 10 turnovers. These observations distinguish SPL from DNA photolyases. To the best of our knowledge, SPL represents an unprecedented example of a major DNA repair enzyme that cannot effectively repair its substrate lesion within the normal DNA conformation adopted in growing cells. Factors such as other DNA binding proteins may have to cooperate with SPL to enable the efficient SP repair in germinating spores. Moreover, if SP is produced in growing cells, the SP repair may be slow and potentially problematic.

Spore photoproduct within DNA is a surprisingly poor substrate for its designated repair enzyme: The spore photoproduct lyase

254th National Meeting (2017)

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