tRNA-dependent addition of amino acids to cell wall and membrane components.

The objective of the present review is to provide an insight into modifications of microbial cell walls and membrane constituents by using the aminoacyl-tRNA as amino acid donor. In bacteria, phospholipids are modified by Multiple peptide resistance Factor enzymes and peptidoglycan precursors by so called fem ligases. Although these modifications were thought to be restricted to procaryotes, we discovered enzymes that modify ergosterol (the main component of fungal membrane) with glycine and aspartate.

RNA-dependent synthesis of ergosteryl-3β-O-glycine in Ascomycota expands the diversity of steryl-amino acids.

A wide range of bacteria possess virulence factors such as aminoacyl-tRNA transferases (ATTs) that are capable of rerouting aminoacyl-transfer RNAs away from protein synthesis to conjugate amino acids onto glycerolipids. We recently showed that, although these pathways were thought to be restricted to bacteria, higher fungi also possess ergosteryl-3β-O-L-aspartate synthases (ErdSs), which transfer the L-Asp moiety of aspartyl-tRNA onto the 3β-OH group of ergosterol (Erg), yielding ergosteryl-3β-O-L-aspartate (Erg-Asp). Here, we report the discovery, in fungi, of a second type of fungal sterol-specific ATTs, namely, ergosteryl-3β-O-glycine (Erg-Gly) synthase (ErgS).

RNA-dependent sterol aspartylation in fungi.

Diverting aminoacyl-transfer RNAs (tRNAs) from protein synthesis is a well-known process used by a wide range of bacteria to aminoacylate membrane constituents. By tRNA-dependently adding amino acids to glycerolipids, bacteria change their cell surface properties, which intensifies antimicrobial drug resistance, pathogenicity, and virulence. No equivalent aminoacylated lipids have been uncovered in any eukaryotic species thus far, suggesting that tRNA-dependent lipid remodeling is a process restricted to prokaryotes.

Noncanonical inputs and outputs of tRNA aminoacylation.

The aminoacylation reaction is one of most extensively studied cellular processes. The so-called "canonical" reaction is carried out by direct charging of an amino acid (aa) onto its corresponding transfer RNA (tRNA) by the cognate aminoacyl-tRNA synthetase (aaRS), and the canonical usage of the aminoacylated tRNA (aa-tRNA) is to translate a messenger RNA codon in a translating ribosome. However, four out of the 22 genetically-encoded aa are made "noncanonically" through a two-step or indirect route that usually compensate for a missing aaRS.