Fig. 7. (A) Schematic depiction of catabolic flow of intracellular taurine. Plasma taurine is taken up through the taurine transporter. Cytoplasmic taurine is excreted as such or in the form of bile salts like taurocholate. Cytoplasmic taurine is imported into mitochondria through a putative mitochondrial taurine transporter to be used as a constituent for τm5(s2)U synthesis in mt tRNAs. (B) Possible biosynthetic pathway of τm5(s2)U. Mitochondrial trmE (trmE*) is involved in the initial step of τm5(s2)U synthesis. After some unidentified steps, mitochondrial taurine is directly incorporated into mt tRNAs to build τm5U. Mitochondrial trmU (trmU*) is responsible for 2-thio group synthesis of τm5s2U.