Crystal structure of Nanoarchaeum equitans tyrosyl-tRNA synthetase and its aminoacylation activity toward tRNA^Tyr with an extra guanosine residue at the 5ʹ-terminus

tRNA^Tyr of Nanoarchaeum equitans has a remarkable feature with an extra guanosine residue at the 5ʹ-terminus. However, the N. equitans tRNA^Tyr mutant without extra guanosine at the 5ʹ-end was tyrosylated by tyrosyl-tRNA synthase (TyrRS). We solved the crystal structure of N. equitans TyrRS at 2.80 Å resolution. By comparing the present solved structure with the complex structures TyrRS with tRNA^Tyr of Thermus thermophilus and Methanocaldococcus jannaschii, an arginine substitution mutant of N. equitans TyrRS at Ile200 (I200R), which is the putative closest candidate to the 5ʹ-phosphate of C1 of N. equitans tRNA^Tyr, was prepared. The I200R mutant tyrosylated not only wild-type tRNA^Tyr but also the tRNA without the G-1 residue. Further tyrosylation analysis revealed that the second base of the anticodon (U35), discriminator base (A73), and C1:G72 base pair are strong recognition sites.