Mohamed Atta, Victor Duarte & Philippe Carpentier

The post-transcriptional processing of transfer RNAs (tRNAs) involves a number of events essential for their maturation. Several works point out clear correlations between the absence of a modified nucleoside and human diseases, cellular response to stresses conditions, attesting for the vital importance in cell metabolism. In the light of the high chemical diversity encountered and the complexity of the catalyzed reactions, enzymology of tRNA modifying enzymes is a rich source of fascinating topics in enzymology. Beyond their fascinating chemistry, these enzymes are appealing targets for studies for several reasons, as they offer insight into protein-RNA interactions. They must discriminate between substrate and non-substrate RNA as well as substrate and product nucleosides within the same RNA context, i.e. they must discriminate between multiple tRNAs of globally identical shape. Furthermore, the RNA-modifying enzymes catalyze many sophisticated reactions that merit examination in their own right.

One of my projects is the enzymology of a prokaryotic biosynthetic pathway leading to the formation of the modified nucleoside 2-methylthio-N-6-(cis-hydroxy)isopentenyl adenosine (ms²io⁶A₃₇) and its derivatives which are present at position 37 adjacent to the anticodon of specific tRNAs (figure). The first reaction, which depends on MiaA enzyme, consists in the attachment of the isopentenyl group to N6 nitrogen of adenosine to convert tRNA-A₃₇ into tRNA-i⁶A₃₇. The following steps, which are of particular interest, involve two tRNA-modifying metalloenzymes, MiaB and MiaE, each belonging to its distinct group of metalloproteins, namely the Radical-SAM family for MiaB and the carboxylate-bridged non-heme di-iron enzyme family for MiaE.


Figure: Biosynthetic pathway of ms²io⁶A₃₇.
The enzymes involved in this pathway are MiaA, MiaB, and MiaE. DMAPP, dimethylallyl diphosphate; PPi, pyrophosphate; SAM, S-adenosylmethionine; SAH, S-adenosylhomocysteine; AdoH, 5’-deoxyadenosyl.