Abstract
Phosphorothioate (PT) modification, a sequence-specific modification that replaces the nonbridging oxygen atom with sulfur in a DNA phosphodiester through the gene products of dndABCDE or sspABCD, is widely distributed in prokaryotes. DNA PT modification functions together with gene products encoded by dndFGH, pbeABCD, or sspE to form defense systems that can protect against invasion by exogenous DNA particles. While the functions of the multiple enzymes in the PT system have been elucidated, the exact role of DndE in the PT process is still obscure. Here, we solved the crystal structure of DndE from the haloalkaliphilic archaeal strain Natronorubrum bangense JCM10635 at a resolution of 2.31 Å. Unlike the tetrameric conformation of DndE in Escherichia coli B7A, DndE from N. bangense JCM10635 exists in a monomeric conformation and can catalyze the conversion of supercoiled DNA to nicked or linearized products. Moreover, DndE exhibits preferential binding affinity to nicked DNA by virtue of the R19- and K23-containing positively charged surface. This work provides insight into how DndE functions in PT modification and the potential sulfur incorporation mechanism of DNA PT modification. IMPORTANCE DndABCDE proteins have been demonstrated to catalyze DNA PT modification with the nonbridging oxygen in the DNA sugar-phosphate backbone replaced by sulfur. In the PT modification pathway, DndA exerts cysteine desulfurase activity capable of catalyzing the mobilization of sulfur from l-cysteine, which involves the ion-sulfur cluster assembly of DndC. This is regarded as the initial step of the DNA PT modification. Moreover, DndD has ATPase activity in vitro, which is believed to provide energy for the oxygen-sulfur swap, while the function of DndE is unknown. However, the exact function of the key enzyme DndE remains to be elucidated. By determining the structure of DndE from the haloalkaliphilic strain Natronorubrum bangense JCM10635, we showed that the archaeal DndE adopts a monomer conformation. Notably, DndE can introduce nicks to supercoiled DNA and exhibits a binding preference for nicked DNA; the nicking is believed to be the initial step for DNA to facilitate the sulfur incorporation.