Iron-sulfur biology invades tRNA modification: the case of U34 sulfuration

Sulfuration of uridine 34 in the anticodon of tRNAs is conserved in the three domains of life, guaranteeing fidelity of protein translation. In eubacteria, it is catalyzed by MnmA-type enzymes, which were previously concluded not to depend on an iron-sulfur [Fe-S] cluster. However, we report here spectroscopic and iron/sulfur analysis, as well as in vitro catalytic assays and site-directed mutagenesis studies unambiguously showing that MnmA from Escherichia coli can bind a [4Fe-4S] cluster, which is essential for sulfuration of U34-tRNA. We propose that the cluster serves to bind and activate hydrosulfide for nucleophilic attack on the adenylated nucleoside. Intriguingly, we found that E. coli cells retain s2U34 biosynthesis in the ΔiscUA ΔsufABCDSE strain, lacking functional ISC and SUF [Fe-S] cluster assembly machineries, thus suggesting an original and yet undescribed way of maturation of MnmA. Moreover, we report genetic analysis showing the importance of MnmA for sustaining oxidative stress.