Roles of tRNA in cell wall biosynthesis

Abstract

Recent research into various aspects of bacterial metabolism such as cell wall and antibiotic synthesis, degradation pathways, cellular stress, and amino acid biosynthesis has elucidated roles of aminoacyl-transfer ribonucleic acid (aa-tRNA) outside of translation. Although the two enzyme families responsible for cell wall modifications, aminoacyl-phosphatidylglycerol synthases (aaPGSs) and Fem, were discovered some time ago, they have recently become of intense interest for their roles in the antimicrobial resistance of pathogenic microorganisms. The addition of positively charged amino acids to phosphatidylglycerol (PG) by aaPGSs neutralizes the lipid bilayer making the bacteria less susceptible to positively charged antimicrobial agents. Fem transferases utilize aa-tRNA to form peptide bridges that link strands of peptidoglycan. These bridges vary among the bacterial species in which they are present and play a role in resistance to antibiotics that target the cell wall. Additionally, the formation of truncated peptides results in shorter peptide bridges and loss of branched linkages which makes bacteria more susceptible to antimicrobials. A greater understanding of the structure and substrate specificity of this diverse enzymatic family is necessary to aid current efforts in designing potential bactericidal agents. These two enzyme families are linked only by the substrate with which they modify the cell wall, aa-tRNA; their structure, cell wall modification processes and the physiological changes they impart on the bacterium differ greatly.

FIGURE 1

Aminoacyl-phosphatidylglycerol synthase (aaPGS) modifications contribute to the charge of the cell wall. Shown are the known modifications and the resulting charge of the modified phosphatidylglycerol (PG).

FIGURE 2

Formation of the peptidoglycan layer in some bacterial species involves peptide bridges formed by Fem proteins. Shown is a schematic of cell wall biogenesis, below each precursor the amino acid specificity of the Fem enzyme(s) found in the organism listed to the left is indicated in the same color text as the precursor used by the enzyme for amino acid transfer. For Enterococcus faecalis the distinction of which lipid precursor that is preferred by the enzyme has not been determined. For Weissella viridescens, FemX only adds the first amino acid, the other two are added by a second unknown enzyme or enzymes.

FIGURE 3

Structural comparisons of Staphylococcus aureus FemA and Weissella viridescens FemX. (a) Sequence and structure of FemA with an additional coiled coil absent in FemX. (b) Domains 1A and 1B of W. viridescens FemX. The two proteins can be structurally compared to the catalytic domains of the histone acetyltransferase of Tetrahymena thermophila (c), and the serotonin acetyltransferase of Ovis ovaries (d). From a review published by Mainardi et al.

FIGURE 4

tRNA isoacceptors predominantly utilized by Fem proteins in peptidoglycan biosynthesis. Differences found to be important for Fem recognition or features that are detrimental to protein synthesis in red. In blue are the differences between the two co-purified tRNA species shown to be preferred substrates of Staphylococcus epidermidis FemA.