InterPro domain: IPR035107

tRNA-cytidine(32) 2-sulfurtransferase (also known as 2-thiocytidine tRNA biosynthesis protein TtcA) is required for the thiolation of cytidine in position 32 of tRNA, to form 2-thiocytidine (s(2)C32). The modified nucleoside 2-thiocytidine (s(2)C) has so far been found in tRNA from archaea and bacteria. The TtcA protein family is characterised by the existence of both a PP-loop and a Cys-X(1)-X(2)-Cys motif in the central region of the protein but can be divided into two distinct groups based on the presence and location of additional Cys-X(1)-X(2)-Cys motifs in terminal regions of the sequence. Mutant analysis showed that both cysteines in this central conserved Cys-X(1)-X(2)-Cys motif are required for the formation of s(2)C [ 1 ].

The PP-loop motif appears to be a modified version of the P-loop of nucleotide binding domain that is involved in phosphate binding [ 2 ]. Named PP-motif, since it appears to be a part of a previously uncharacterised ATP pyrophophatase domain. ATP sulfurylases, Escherichia coli NtrL, and Bacillus subtilis OutB consist of this domain alone. In other proteins, the pyrophosphatase domain is associated with amidotransferase domains (type I or type II), a putative citrulline-aspartate ligase domain or a nitrilase/amidase domain.

Cytoplasmic tRNA 2-thiolation protein 1 (also known as Ncs6/Tuc1 in budding yeasts) is responsible for 2-thiolation of mcm5S2U at tRNA wobble positions of tRNA(Lys), tRNA(Glu) and tRNA(Gln) [ 3 , 4 ]. It directly bind tRNAs and probably acts by catalysing adenylation of tRNAs, an intermediate required for 2-thiolation. Its fission yeast homologue, Ctu1 forms a complex with Ctu2 (Ncs2/Tuc2 homologue) and serves as a putative enzyme for the formation of 2-thiouridine [ 5 ]. This family also includes tRNA-5-methyluridine(54) 2-sulfurtransferase, which catalyzes the 2-thiolation of 5-methyluridine residue at position 54 in the T loop of tRNAs, leading to 5-methyl-2-thiouridine [ 6 ].

This entry represents a group of tRNA thiolation proteins. It includes the tRNA-cytidine(32) 2-sulfurtransferase ( IPR012089 ), and Cytoplasmic tRNA 2-thiolation protein 1 ( IPR000541 ).

1. The conserved Cys-X1-X2-Cys motif present in the TtcA protein is required for the thiolation of cytidine in position 32 of tRNA from Salmonella enterica serovar Typhimurium. J. Bacteriol. 186, 750-7
2. A P-loop-like motif in a widespread ATP pyrophosphatase domain: implications for the evolution of sequence motifs and enzyme activity. Proteins 20, 347-55
3. Thio-modification of yeast cytosolic tRNA requires a ubiquitin-related system that resembles bacterial sulfur transfer systems. J. Biol. Chem. 283, 27469-76
4. The conserved Wobble uridine tRNA thiolase Ctu1-Ctu2 is required to maintain genome integrity. Proc. Natl. Acad. Sci. U.S.A. 105, 5459-64
5. Mechanistic characterization of the sulfur-relay system for eukaryotic 2-thiouridine biogenesis at tRNA wobble positions. Nucleic Acids Res. 37, 1335-52
6. Nonredox thiolation in tRNA occurring via sulfur activation by a [4Fe-4S] cluster. Proc. Natl. Acad. Sci. U.S.A. 114, 7355-7360