Crystallographic Study of the 2-Thioribothymidine-synthetic Complex TtuA-TtuB from Thermus Thermophilus.

The ubiquitin-like protein TtuB is a sulfur carrier for the biosynthesis of 2-thioribothymidine (s2T) at position 54 in some thermophilic bacterial tRNAs. TtuB captures a S atom at its C-terminus as a thiocarboxylate and transfers it to tRNA by the transferase activity of TtuA. TtuB also functions to suppress s2T formation by forming a covalent bond with TtuA. To explore how TtuB interacts with TtuA and switches between these two different functions, high-resolution structure analysis of the TtuA–TtuB complex is required. In this study, the TtuA–TtuB complex fromThermus thermophiluswas expressed, purified and crystallized. To mimic the thiocarboxylated TtuB, the C-terminal Gly residue was replaced with Cys (G65C) to obtain crystals of the TtuA–TtuB complex. A Zn-MAD data set was collected to a resolution of 2.5 Å. MAD analysis successfully determined eight Zn sites, and a partial structure model composed of four TtuA–TtuB complexes in the asymmetric unit was constructed.