Molecular basis of cobalamin-dependent RNA modification

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• Queuosine (Q) is a vital hypermodified nucleoside found in the tRNA wobble position, crucial for accurate protein synthesis.
• The enzyme epoxyqueuosine (oQ) reductase (QueG) catalyzes the final step in Q biosynthesis but its mechanism remained elusive.
• QueG is a unique cobalamin (Cbl)-dependent enzyme, incorporating a [4Fe-4S] cluster and catalyzing an unprecedented epoxide reduction.

Purpose of the Study:

• To determine the high-resolution structures of Bacillus subtilis QueG.
• To elucidate the mechanism by which QueG performs epoxide reduction using cobalamin.
• To understand how QueG recognizes specific tRNAs and positions substrates for catalysis.

Main Methods:

• X-ray crystallography was used to obtain high-resolution structures of QueG, including complexes with tRNA.
• Protein film electrochemistry was employed to study the redox properties of the enzyme's cofactors.
• Biochemical assays were performed to investigate enzyme activity and substrate interactions.

Main Results:

• Structural analysis revealed a HEAT-like domain in QueG responsible for anticodon recognition and substrate positioning.
• QueG positions the substrate directly above the cobalamin cofactor, suggesting a covalent intermediate.
• Electrochemical data indicated that the [4Fe-4S] clusters facilitate cobalamin reduction, activating it for catalysis.
• The study provides the first structural and mechanistic insights into Cbl-dependent nucleic acid modification via epoxide reduction.

Conclusions:

• QueG utilizes a unique structural mechanism involving a HEAT-like domain and cobalamin cofactor for tRNA modification.
• The enzyme's [4Fe-4S] clusters play a critical role in activating the cobalamin for the epoxide reduction reaction.
• This work significantly advances the understanding of cobalamin-dependent enzymes and nucleic acid hypermodification.