Table 1.
Non-canonical functions of transfer RNAs (tRNAs), their fragments, and mimics. Table based mainly on references [53,54,55,56,57,58,59,60,61,62]. See text for additional references. miRNA = microRNA; siRNA = small interfering RNA.
| Non-Canonical tRNA Properties | Molecules Involved | Functions |
|---|---|---|
| Modification of Macromolecules | ||
| Lipid aminoacylation | Phosphatidylglycerol synthetases | Protects against some antibiotics |
| Enzymatic modification of peptidoglycans | MurM, Fem aminoacyl transferases | Modifies peptidoglycan synthesis in cell wall; alters protein binding |
| Enzymatic modification of amino terminus of proteins | Leucyl/Phenylalanyl transferases; Arg-tRNA transferase | Targets proteins for degradation |
| Enzymatic modification of peptides | AlbC, PacB, VlmA, DhpH-C peptidyltransferase | Intracellular signaling; Metal chelation; Antibiotic formation |
| Synthesis of Small Metabolites | ||
| Dipeptide synthesis | Diketopiperazines | Metal chelation |
| Tetrapyrrole synthesis | Heme; Chlorophyll | Energy transduction |
| Aminoacylation of tRNA | Gln; Asn; fMet; Cys; Sec | Makes up for missing tRNA |
| Regulatory Processes | ||
| Bind directly to ribosome to modify activity | Deacylated tRNA | Stringent response: Form alarmones, ppGpp and pppGpp |
| DNA replication initiation | ColE1-type plasmids | Control of plasmid replication |
| Riboswitch control | Operons ; T-box | Transcription attenuation |
| Metabolism | Siderophores; Virulence factors | Stress responses; Aerobic/respiratory metabolism; Gene expression |
| Stress responses | Proteins and polysomes involved in siRNA binding & miRNA binding | Translation regulation |
| tRNA Mimicry | ||
| Translation regulation | tmRNA (small transfer-messenger RNA; SsrA or 10Sa RNA) & tmRNA-specific binding protein (SmpB or small protein B). | Rescues stalled ribosomes |
| “t-elements” (tRNA-like elements) in mRNAs | 3′ and 5′ ends of mitochondrial mRNA | Sites for RNAase P and RNAase Z activation of mRNA? |
| Short Interspersed nuclear elements (SINEs) | Genome (non-coding DNA) | Retrotransposons |
| Viral tRNA mimics | Viral mRNA | Hijacking translation machinery |
| tRNA Fragments | ||
| tRNA halves | Outer membrane vesicles | Stress responses? Bacterial signaling? |
| tRNA-derived fragments (tRFs) | Ribosomal proteins; mRNAs; RNA-binding proteins | Interferes w/with peptidyl transferase; Act as miRNAs or siRNAs in post-transcriptional gene silencing; Buffer regulatory RNAs; Sequester RNA-binding proteins |