Conserved Bases in the TΨC Loop of tRNA Are Determinants for Thermophile-specific 2-Thiouridylation at Position 54

2-Thioribothymidine (s(2)T) is a post-transcriptionally modified nucleoside of U54 specifically found in thermophilic bacterial tRNAs. The 2-thiocarbonyl group of s(2)T54 is known to be responsible for the thermostability of tRNA. The s(2)T54 content in tRNA varies depending on the cultivation temperature, a feature that confers thermal adaptation of protein synthesis in Thermus thermophilus. Little is known about the biosynthesis of s(2)T, including the sulfur donor, modification enzyme, and the tRNA structural requirements. To characterize 2-thiolation at position 54 in tRNA, we constructed an in vivo expression system using tRNA(Asp) with an altered sequence and a host-vector for T. thermophilus. We were able to detect in vivo activity of s(2)T54 thiolase using phenyl mercuric gel electrophoresis followed by Northern hybridization. 2-Thiolation at position 54 was identified in the precursor form of the tRNA, indicating that 2-thiolation precedes tRNA processing. To ascertain the elements that determine 2-thiolation in tRNA, systematic site-directed mutagenesis was carried out using the tRNA(Asp) gene. Conserved residues C56 and A58 were identified as major determinants of 2-thiolation, whereas tertiary interaction between the T and D loops and non-conserved nucleosides in the T loop were revealed not to be important for the reaction.