# EF-Tu
**EF-Tu** (elongation factor thermo unstable, also called EF1A) is a bacterial GTP-binding protein that delivers aminoacyl-tRNAs to the ribosome during the elongation phase of protein synthesis. It forms a ternary complex with GTP and an aminoacyl-tRNA (aa-tRNA·EF-Tu·GTP), escorts the tRNA to the ribosomal A site, and hydrolyzes its bound GTP after the tRNA's anticodon is matched against the mRNA codon, releasing the tRNA for peptide-bond formation[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3763471/) • [2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3158919/). Homologous factors perform the same role in archaea (aEF-1A) and eukaryotes (eEF1A)[3](https://onlinelibrary.wiley.com/doi/full/10.1002/pro.4393).
| Key fact | Detail |
|---|---|
| Core function | Carries aminoacyl-tRNA to the ribosomal A site as part of the ternary complex aa-tRNA·EF-Tu·GTP; GTP hydrolysis releases the tRNA[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3763471/) |
| Abundance | With 10 or more copies per ribosome, EF-Tu is the most abundant protein in *E. coli*[4](https://academic.oup.com/nar/article/48/3/1068/5644990) |
| Size and domains | *E. coli* EF-Tu is 394 amino acids in three domains: G (residues 1–208, GTP binding and hydrolysis), D-II (209–298, binds the tRNA acceptor stem and amino acid), and D-III (299–394)[3](https://onlinelibrary.wiley.com/doi/full/10.1002/pro.4393) |
| GTPase activation | Codon recognition orders the Switch I loop and positions catalytic His84 to coordinate the water attacking the γ-phosphate of GTP, via contact with A2662 of the 23S rRNA Sarcin-Ricin Loop[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3763471/) |
| Accuracy contribution | Two-step tRNA selection (initial selection plus proofreading) is largely responsible for the ribosome's 99.99% translation accuracy; bacterial missense error is roughly 3,000:1 or better[5](https://www.ncbi.nlm.nih.gov/books/NBK26829/) • [6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5095583/) |
| Homologs | aEF-1A in archaea and eEF1A in eukaryotes; mammals have two variants, eEF1A1 (most tissues) and eEF1A2 (muscle and brain)[3](https://onlinelibrary.wiley.com/doi/full/10.1002/pro.4393) • [7](https://pmc.ncbi.nlm.nih.gov/articles/PMC10074971/) |
| Evolution | EF-Tu and the initiation factor IF2 descend from a common IF2-like GTPase ancestor that predates the last universal common ancestor[3](https://onlinelibrary.wiley.com/doi/full/10.1002/pro.4393) |
## Role in the elongation cycle
Each round of bacterial translation elongation begins when the ternary complex of aminoacyl-tRNA, EF-Tu, and GTP associates with the ribosome. This initial binding is codon-independent and is mediated mainly by contacts between EF-Tu and the ribosomal protein L7/12[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3158919/). Once the tRNA's anticodon pairs with the mRNA codon in the A site, recognition of the codon causes a conformational shift in the 30S subunit that docks EF-Tu against the Sarcin-Ricin Loop (SRL) of the 23S rRNA, triggering GTP hydrolysis[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3763471/) • [8](https://pmc.ncbi.nlm.nih.gov/articles/PMC7483604/).
Hydrolysis converts EF-Tu to its GDP-bound form, which releases the aminoacyl-tRNA so its acceptor end can swing into the peptidyl transferase center and the amino acid can be added to the growing chain. tRNA binds only the GTP-bound state of EF-Tu, so hydrolysis itself causes dissociation of the complex[9](https://people.sabanciuniv.edu/atilgan/sbqual/Textbooks/Petsko_Protein%20Structure%20and%20Function.pdf). The factor is then recharged: EF-Ts, its guanine nucleotide exchange factor, accelerates the release of GDP so GTP can bind again[10](https://hdl.handle.net/11858/00-001M-0000-0026-DBF2-D).
## Mechanism of GTPase activation
A central question has been how the ribosome switches EF-Tu's GTPase on at the right moment and prevents premature hydrolysis. In isolated EF-Tu, a hydrophobic gate built from Val20 of the P-loop and Ile60 of Switch I restricts access of the catalytic histidine His84 to the water molecule that attacks the γ-phosphate of GTP[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3763471/).
The answer came from a 3.2 Å crystal structure of EF-Tu and aminoacyl-tRNA bound to the 70S ribosome with the GTP analog GDPCP (β-γ-methyleneguanosine 5′-triphosphate), which stalls the complex before hydrolysis. In this GTPase-competent state the Switch I loop is ordered in the GTPase center, and His84 coordinates the nucleophilic water in position for in-line attack on the γ-phosphate. The activated conformation depends on a conserved interaction of His84 with A2662 of the Sarcin-Ricin Loop, and the structure suggests a mechanism for GTPase activation that applies to translational GTPases generally[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3763471/). Consistent with this, structures of EF-Tu and EF-G trapped pre-hydrolysis show the conserved histidine contacting the phosphate group of base A2662[11](https://onlinelibrary.wiley.com/doi/full/10.1002%2Fbip.22832).
## Fidelity and proofreading
EF-Tu is a major contributor to the accuracy of genetic-code translation. Codon-anticodon hydrogen bonding alone makes a cognate (correct) ternary complex only marginally more stable, by several kBT, than a near-cognate one, which would predict a selectivity of roughly 100:1; actual bacterial translation accuracy is about 3,000:1 or greater[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5095583/). The gap is closed by splitting selection into two steps separated by the irreversible GTP hydrolysis: initial selection before hydrolysis, and proofreading after it, when incorrect tRNAs are preferentially rejected during accommodation[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC3158919/). These two delays cause most incorrectly bound tRNAs, and a significant number of correctly bound ones, to leave the ribosome unused, and this two-step mechanism is largely responsible for the ribosome's 99.99% accuracy[5](https://www.ncbi.nlm.nih.gov/books/NBK26829/).
EF-Tu's own dissociation rate is tuned into this scheme. [Molecular dynamics](https://www.edgepedia.org/molecular-dynamics) and kinetic modeling indicate that if EF-Tu dissociates too slowly, proofreading disappears because the factor blocks aa-tRNA dissociation regardless of codon; if it dissociates too fast, even cognate tRNAs are rejected more often. Efficient proofreading requires a dissociation rate slow enough to assist accommodation yet fast enough to allow rejection of near-cognate tRNAs[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5095583/).
The factor also appears to check the amino acid itself. As the GTP-bound form escorts aminoacyl-tRNAs, it apparently double-checks the correspondence between amino acid and tRNA and rejects mismatches, plausibly because correct matches fall within a narrow range of binding affinity to EF-Tu while incorrect ones bind too strongly or too weakly[5](https://www.ncbi.nlm.nih.gov/books/NBK26829/). Consistent with a defined binding envelope, the amino acid-binding pocket accommodates amino acids as large as tryptophan[12](https://pmc.ncbi.nlm.nih.gov/articles/PMC6851308/), and EF-Tu binds aminoacyl-tRNAs aminoacylated by class II synthetases on the 3′-OH, which form the more stable 3′-isomers[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC3716924/) • [18](https://link.springer.com/article/10.1186/1745-6150-8-17).
## Structure and tRNA binding
All tRNA molecules, despite their sequence differences, share the same overall structure and can bind the same EF-Tu[9](https://people.sabanciuniv.edu/atilgan/sbqual/Textbooks/Petsko_Protein%20Structure%20and%20Function.pdf). Binding involves all three domains: the G domain is essential for GTP hydrolysis and protein function, while D-II contacts the acceptor stem of the tRNA and the amino acid[3](https://onlinelibrary.wiley.com/doi/full/10.1002/pro.4393). Because EF-Tu must bind every correctly charged elongator tRNA but reject mischarged ones, engineering tRNAs for cell-free synthesis often targets EF-Tu binding; a hybrid tRNA designed with the T-stem motif of *E. coli* tRNAGlu and the D-arm motif of tRNAPro1 was built specifically to improve binding to EF-Tu[4](https://academic.oup.com/nar/article/48/3/1068/5644990).
## Evolution
EF-Tu belongs to the family of translational GTPases, essential and highly conserved enzymes that also include EF-G and initiation factor 2[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC3763471/). Phylogenetic modeling and ancestral sequence reconstruction indicate that EF-Tu and IF2 evolved from a common ancestor in the [last universal common ancestor](https://www.edgepedia.org/last-universal-common-ancestor) (LUCA), and that this ancestor was an IF2-like GTPase. The two proteins retain homologous tRNA-binding domains (IF2 C2 and EF-Tu D-II, both OB-folds with only 13% sequence identity) and conserved GTPase motifs; the reconstructed common ancestor's GTP-binding domain shares about 85% mean identity with the ancestral IF2 but only about 43% with the ancestral EF-Tu[3](https://onlinelibrary.wiley.com/doi/full/10.1002/pro.4393). The genes for EF-Tu and EF-G are the products of an ancient gene duplication that appears to predate the divergence of all extant organismal lineages[14](https://pmc.ncbi.nlm.nih.gov/articles/PMC38819/?page=0).
## Homologs and non-canonical roles
The eukaryotic counterpart eEF1A delivers all aminoacyl-tRNAs except initiator and selenocysteine tRNAs to the ribosome, and is among the most abundant cellular proteins[15](https://www.jbc.org/article/S0021925820601454/pdf). Mammals carry two variants, eEF1A1 and eEF1A2, which are 97% homologous, expressed in a mutually exclusive pattern (eEF1A2 in muscle and brain, eEF1A1 elsewhere), and encoded on different chromosomes[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC10074971/). Bacterial EF-Tu also has roles beyond translation: it executes various functions in eukaryotes and has moonlighting roles in [microbial pathogenesis](https://www.edgepedia.org/microbial-pathogenesis), which is why reviews of its functions use the older prokaryotic name EF-Tu alongside the systematic name EF1A[16](https://pmc.ncbi.nlm.nih.gov/articles/PMC6822514/).
## Practical use
Because EF-Tu, EF-Ts, and EF-G are individually purified components of defined translation systems, they are supplied in reconstituted cell-free protein expression platforms such as the PURE system, where each translation factor can be modified or replaced at known concentrations[17](https://www.neb.com/en-us/applications/protein-expression/cell-free-protein-expression) • [4](https://academic.oup.com/nar/article/48/3/1068/5644990).
## References
1. [The mechanism for activation of GTP hydrolysis on the ribosome (PMC3763471)](https://pmc.ncbi.nlm.nih.gov/articles/PMC3763471/)
2. [Evolutionary optimization of speed and accuracy of decoding on the ribosome (PMC3158919)](https://pmc.ncbi.nlm.nih.gov/articles/PMC3158919/)
3. [Fer et al. (2022). Early divergence of translation initiation and elongation factors. Protein Science.](https://onlinelibrary.wiley.com/doi/full/10.1002/pro.4393)
4. [Hammerling, Krüger & Jewett (2020). Strategies for in vitro engineering of the translation machinery. Nucleic Acids Research.](https://academic.oup.com/nar/article/48/3/1068/5644990)
5. [From RNA to Protein. Molecular Biology of the Cell, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK26829/)
6. [How EF-Tu can contribute to efficient proofreading of aa-tRNA by the ribosome (PMC5095583)](https://pmc.ncbi.nlm.nih.gov/articles/PMC5095583/)
7. [The eEF1 family of mammalian translation elongation factors (PMC10074971)](https://pmc.ncbi.nlm.nih.gov/articles/PMC10074971/)
8. [Cryo-EM of elongating ribosome with EF-Tu·GTP elucidates tRNA proofreading (PMC7483604)](https://pmc.ncbi.nlm.nih.gov/articles/PMC7483604/)
9. [GTPase Switches: Protein Synthesis (Petsko & Ringe, Protein Structure and Function)](https://people.sabanciuniv.edu/atilgan/sbqual/Textbooks/Petsko_Protein%20Structure%20and%20Function.pdf)
10. [Functions and Regulation of Translation Elongation Factors (hdl 11858/00-001M-0000-0026-DBF2-D)](https://hdl.handle.net/11858/00-001M-0000-0026-DBF2-D)
11. [Maracci & Rodnina (2016). Review: Translational GTPases. Biopolymers.](https://onlinelibrary.wiley.com/doi/full/10.1002%2Fbip.22832)
12. [Chiral checkpoints during protein biosynthesis (PMC6851308)](https://pmc.ncbi.nlm.nih.gov/articles/PMC6851308/)
13. [The mechanistic and evolutionary aspects of the 2′- and 3′-OH paradigm in biosynthetic machinery (PMC3716924)](https://pmc.ncbi.nlm.nih.gov/articles/PMC3716924/)
14. [The root of the universal tree and the origin of eukaryotes based on elongation factor phylogeny (PMC38819)](https://pmc.ncbi.nlm.nih.gov/articles/PMC38819/?page=0)
15. [eEF1A: Thinking Outside the Ribosome. Journal of Biological Chemistry.](https://www.jbc.org/article/S0021925820601454/pdf)
16. [The Diverse Functional Roles of Elongation Factor Tu (EF-Tu) in Microbial Pathogenesis (PMC6822514)](https://pmc.ncbi.nlm.nih.gov/articles/PMC6822514/)
17. [Cell-Free Protein Expression, New England Biolabs](https://www.neb.com/en-us/applications/protein-expression/cell-free-protein-expression)
18. [Client Challenge](https://link.springer.com/article/10.1186/1745-6150-8-17)
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*Topic: Encyclopedia › Life and health › Biological foundations › RNA and gene regulation › RNA processing, modification, and translation › Transfer RNA, ribosomal RNA, and translation › Translation factors › Aminoacyl-tRNA delivery factors (EF-Tu/eEF1 family)*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license