EF-Tu, Elongation factor

The second phase of protein synthesis is the elongation cycle. This phase begins with the insertion of an aminoacyl-lRKA into the empty A site on the ribosome. The particular species inserted depends on the mRNA codon in the A site. The cognate ami noacyl-tKN A does not simply leave the synthetase and diffuse to the A site. Rather, it is delivered to the A site in association with a 4. Tkd protein called elongation factor Tu (EF-Tu). Elongation factor Tu, another member of the G-protein family, requires GTP to bind aminoacyl-tRNA (Figure 30.23) and to bind the ribosome. The binding of... 

Elongation Functional 70 S ribosomes aminoacyl-tRNAs GTP, Mg2+, EF-Tu, EF-Ts, and EF-G elongation factors Functional 80 S ribosomes aminoacyl-tRNAs GTP, Mg2+, EF-1, EF-1, and EF-2 elongation factors... 

This prevents further ineorporation of aminoaeyl-tRNA by bloeking the binding of EF-Tu GTP. Like the tetraeyelines, fusidie aeid owes its seleetive antimierobial aetion to active uptake by bacteria and exclusion fiom manunalian cells. The equivalent elongation factor in mammalian cells, EF-2 is susceptible to fusidie acid in cell-free systems. 

EFTsNT A UBA-like domain with a clear role outside of ubiquitin binding is found at the N-terminus of EF-Ts proteins. The relationship of this region to genuine UBA domains is well established as there is a structure of full-length EF-Ts available [67]. Nevertheless, this domain is widespread in bacteria and archaea, which obviously lack a proper ubiquitin system. The physiological role of the EFTsNT domain is rather in the binding to the elongation factor EF-Tu, which has no resemblance to ubiquitin. 

In addition to the ribosomal proteins, the two initiation factors IF-1 (Pon et ai, 1979) and IF-3 (Brauer and Wittmann-Liebold, 1978), the elongation factor EF-Tu (Arai et ai, 1980), and the two proteins NS-1 and NS-2 (Mende et ai, 1978), which bind to ribosomes and to DNA, have been completely sequenced (Table III). 

Figure 7 The direct and indirect pathways of tRNA asparaginylation. The direct pathway consists of charging by AsnRS on tRNA " of free Asn formed with asparagine synthetase A or B. The Asn-tRNA " binds the EF-Tu factor in bacteria (or EF-1A in eukaryotes and archaea) to be carried to the ribosome, in the indirect pathway, a nondiscriminating AspRS (ND-AspRS) charges Asp on tRNA " Asp-tRNA " does not bind the eiongation factor but is converted by the tRNA-dependent trimeric amidotransferase GatCAB into Asn-tRNA ", which binds the EF-Tu factor and is carried to the ribosome where it is used for polypeptide chain elongation.

Upon being formed by the aaRSs, aa-tRNAs are trapped by the elongation factor EF-Tu in prokaryotes and EF-IA in eukaryotes and archaea and carried to the ribosome where they are used for elongation of the... 

Fig. 5.12. Structure of the G-domain of the elongation factor EF-Tu from T. ther-mophilus with bonnd GppNHp, according to Berchthold et al., (1993). The non-hydrolysable analog GppNHp, the P loop and the switch regions I and II are shown, which play an important role in transition from the inactive GDP form to the active GTP form (see also 5.5.6 and 9.2.1). MOLSKRIPT representation according to Kranhs, (1991).

Functional 70S ribosome (initiation complex) Aminoacyl-tRNAs specified by codons Elongation factors (EF-Tu, EF-Ts, EF-G)... 

The third stage of protein synthesis is elongation. Again, our initial focus is on bacterial cells. Elongation requires (1) the initiation complex described above, (2) aminoacyl-tRNAs, (3) a set of three soluble cytosolic proteins called elongation factors (EF-Tu, EF-Ts, and EF-G in bacteria), and (4) GTP. Cells use three steps to add each amino acid residue, and the steps are repeated as many times as there are residues to be added. 

The elongation cycle in eukaryotes is quite similar to that in prokaryotes. Three eukaryotic elongation factors (eEFla, eEFljSy, and eEF2) have functions analogous to those of the bacterial elongation factors (EF-Tu, EF-Ts, and EF-G, respectively). Eukaryotic ribosomes do not have an E site uncharged tRNAs are expelled directly from the P site. 

Monomeric G proteins. An entirely different class of G proteins was discovered when it was found that the small 189-residue, 21-kDa protein products of the human oncogenes and proto-oncogenes known as ras are monomeric G proteins.186 197-200 There are over 80 related proteins of this group of nine families.2003 They include the much larger elongation factor EF-Tu, which functions in protein synthesis (Chapter 29). 

Elimination reactions 526, 530, 677—690 beta, of cystine residues 85 conjugative 689 decarboxylative 689 facilitation by carbonyl group 681 of y substituent 746 of PLP-dependent enzymes 742 reversibility 690 Ellman s reagent 125,125s Elongation factor EF-Tu 558 Elongin complex 564... 

Codon-specific binding of an aminoacyl-tRNA (decoding). The binding of an aminoacyl-tRNA to the A site of the 70S or 80S initiation complex depends upon a protein called elongation factor Tu (EF-Tu... 

A suppressor of frame-shift mutations in Salmonella is a tRNA containing at the anticodon position the nucleotide quartet CCCC instead of the usual CCC triplet anticodon.442 443 It has eight unpaired bases in the anticodon loop instead of the usual seven. Other frame-shift repressor tRNAs have been identified in E. cob, 444 Salmonella, and yeast.445 Not all suppressor genes encode tRNAs. For example, a UGA suppressor from E. coli is a mutant 16S rRNA from which C1054 has been deleted.446 A general nonsense suppressor in yeast is homologous to yeast elongation factor EF-la as well as to E. coli EF-Tu.447... 

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