# Selective Chemical Manipulation of Mannosyl‐ and Galactosyl‐Queuosine in tRNAs of Living Cells

## TL;DR

This study shows that enzymes add sugars to specific sites on tRNA molecules with high precision, suggesting these modifications play important roles in decoding genetic information.

## Contribution

The study demonstrates precise enzymatic addition of sugars to tRNA using chemically manipulated queuine derivatives.

## Key findings

- Sugar-modifying enzymes attach sugars to tRNA with high regio- and chemoselectivity.
- Analysis of modified tRNAs reveals specific attachment of galactose and mannose moieties.
- The sugar modifications likely play decisive roles in the decoding process of tRNA.

## Abstract

Queuosine is a hypermodified nucleoside found in the anticodon loop of tRNAs decoding the amino acids His, Asn, Tyr, and Asp. In tRNATyr, the homoallylic 2‐hydroxyl group of the cyclopentene ring carries an additional β‐galactose, while in tRNAAsp, an α‐mannose is connected to the allylic 3‐hydroxyl group. Although these additional sugar modifications were found in the tRNAs of many animals, including humans, their purpose remains unknown. Recently, the Q‐tRNA galatosyl‐ and mannosyltransferase (QTGAL and QTMAN) that attach the galactose and mannose moieties to the different OH groups of the Q‐base have been identified, but the function of this transfer reaction remains unclear. Here, we performed feeding experiments with synthetic deoxyqueuine derivatives that lack either the allylic, the homoallylic, or both hydroxyl groups. Analysis of the queuosine derivatives that are found in the corresponding tRNAs shows that the glycosylases attach the sugar with precise specificity, suggesting decisive functions of the galactose and mannose sugar moieties in the decoding process.

The Q‐base in the anticodon loop of certain eukaryotic tRNA is further mannosylated and galactosylated. Using feeding of chemically manipulated queuine derivatives, we show that the sugar‐modifying enzymes perform the modification chemistry with high regio‐ and chemoselectivity.