Figure 1.

Modification pathway of U34. Only steps relevant to this work are shown as understood. The first stage in the modification of U34 at position 5 is controlled by MnmE and GidA, but it is unclear how many steps precede the formation of cmnm⁵U. The cmnm⁵ group is then rearranged in two steps catalyzed by MnmC to synthesize the mnm⁵ side chain in some tRNAs. U34 may undergo thiolation at position 2 mediated by MnmA, IscS and other proteins (see text). It is worthy to mention that the mnmC gene is not evolutionarily conserved, which explains for the absence of mnm⁵s²U in yeast mitochondrial tRNAs.