Replacing Pro240 with Gly in hSMUG1 increases uracil excision in R-loop but abolish 5-hydroxymethyluracil excision in ssDNA

Deamination and oxidation of pyrimidines are two major spontaneously damaging events in DNA. The DNA glycosylase SMUG1 initiates repair of deaminated or oxidized bases by breaking the glycosidic bond between the damaged base and deoxyribose, however, has been less studied than most similar enzymes. In the present study, we investigated the glycosylase activity of hSMUG1 P240G and S26R/E35D mutant proteins, in addition to commercial and in-house purified wild type enzyme, using DNA oligonucleotide substrates of different structure [single-stranded (ss), bubble, and R-loop] with uracil (U) inserted at a specific site, and ssDNA with 5-hydroxymethyluracil (hmU) inserted at a defined site. We demonstrate, for the first time, that hSMUG1 P240G and S26R/E35D retained the glycosylase activity for all substrates except hmU in ssDNA, and showed higher relative activity for uracil in R-loop than bubble DNA. In contrast, wild type hSMUG1 was highly active for hmU in ssDNA and demonstrated higher activity for uracil in bubble than R-loop DNA