Biochemical characterization and mutational studies of a thermostable endonuclease III from Sulfolobus islandicus REY15A

Endonuclease III (EndoIII), which is ubiquitous in bacteria, Archaea and eukaryotes, plays an important role in excising thymine glycol (Tg) from DNA. Herein, we present evidence that an EndoIII from the hyperthermophilic crenarchaeon Sulfolobus islandicus REY15A (Sis-EndoIII) is capable of removing Tg from DNA at high temperature. Biochemical data show that the optimal temperature and pH of Sis-EndoIII are ca.70 degrees C and ca.7.0-8.0, respectively. Furthermore, the recombinant Sis-EndoIII retains relative weak activity without a divalent metal ion, and displays maximum activity in the presence of Mg2+ or Ca2+. Additionally, we first revealed the activation energy (E-a) of 39.7 +/- 4.2 kcal/mol for Sis- EndoIII to remove Tg from dsDNA. As a bifunctional glycosylase, Sis-EndoIII possesses AP lyase activity in addition to glycosylase activity. Additionally, a covalent intermediate is formed between Sis-EndoIII and Tg-containing dsDNA. Mutational studies demonstrate that residues D50, K133 and D151 in Sis-EndoIII are responsible for removal of Tg from dsDNA and K133 and D151 are essential for formation of the covalent intermediate. To our knowledge, it is the first report of Tg excision by crenarchaeal EndoIII, thus augmenting our understanding on archaeal EndoIII function.