Anticodon loop remodeling and D-stem shape drive the specific recognition of ANN-decoding tRNAs for t6A modification.

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• N6-threonylcarbamoyladenosine (t6A) is a vital modification on transfer RNA (tRNA) that ensures accurate protein translation.
• This modification is specifically installed on ANN-decoding tRNAs by the TsaBD complex in bacteria.
• The precise sequence requirements for t6A modification, particularly the 36-UAA-38 motif, have remained structurally uncharacterized.

Purpose of the Study:

• To elucidate the structural mechanisms underlying the sequence specificity of t6A modification on tRNA.
• To understand how the t6A synthase complex recognizes and modifies specific tRNA substrates.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to determine high-resolution structures of the t6A synthase.
• Structures were obtained for both unmodified and t6A-modified tRNA complexes.
• Mutagenesis studies were integrated with structural data.

Main Results:

• The anticodon loop of tRNA adopts a novel zig-zag conformation within the t6A synthase active site.
• tRNA bases U36 and A38 are recognized indirectly through interactions with U32 and the D-stem, explaining specificity.
• A37 is precisely positioned in the active site, while wobble and middle anticodon bases show tolerance to identity.

Conclusions:

• The study reveals the molecular basis for t6A modification's restriction to ANN-decoding tRNAs.
• The findings highlight the importance of indirect readout mechanisms and specific RNA-protein interactions for tRNA modification specificity.
• This work provides critical insights into the structural determinants of translational fidelity.