Ribosome recycling factor

EF-G and the ribosome recycling factor (RRF) catalyze dissociation of the tRNA and the mRNA from the ribosome as well as dissociation of the two ribosomal subunits. Consequently, the ribosome as well as the mRNA are prepared for a new round of protein synthesis. 

The ribosome recycling factor (RRF) is a 21 kDa protein which is involved in the termination step of protein biosynthesis and catalyses the breakdown of the post termination complex into ribosome, tRNA and mRNA. The solution structure of RRF from the hyperthermophilic bacterium Aquifex aeolicus (7 opt = 85°Q was determined by heteronuclear multidimensional NMR spectroscopy, whereas the backbone NMR assignment was recently carried out for RRF from Themotoga maritima and Thermus thermophilus ... 

Abbreviations aa-tRNA Amino-acyl tRNA eLF Eukaryotic translation initiation factor IF Prokaryotic translation initiation factor eEF Eukaryotic translation elongation factor EF Prokaryotic translation elongation factor eRF Eukaryotic translation termination factor (release factor) RF Prokaryotic translation release factor RRF Ribosome recycling factor Rps Protein of the prokaryotic small ribosomal subunit Rpl Protein of the eukaryotic large ribosomal subunit S Protein of the prokaryotic small ribosomal subunit L Protein of the prokaryotic large ribosomal subunit PTC Peptidyl transferase center RNC Ribosome-nascent chain-mRNA complex ram Ribosomal ambiguity mutation RAC Ribosome-associated complex NMD Nonsense-mediated mRNA decay... 

Ribosome recycling factor (RRF) and elongation factor-G (EF-G) are required to recycle the prokaryotic ribosome back to a new round of initiation after termination (Nakamura and Ito 2003). No recycling factor has been identified so far in the cytoplasm of eukaryotic cells. To explain this difference it has been postulated that eukaryotic eRF3 has a dual function ... 

The coding region ends with the protein sequence—that is, there is no stop codon present. In the prokaryotic system the presence of a stop codon would result in the binding of the release factors (Grentzmann et al, 1995 Tuite and Stansfield, 1994) and the ribosome recycling factor (Janosi et al., 1994) to the mRNA-ribosome-protein complexes. This would then lead to the release of the protein by hydrolysis of the peptidyl-tRNA (Tate and Brown, 1992), thereby dissociating the ribosomal complexes (Fig. 4A). A similar mechanism exists in eukaryotic systems (Frolova et al., 1994 Zhouravleva et al., 1995). 

Gong M, Cruz-Vera LR, Yanofsky C. Ribosome recycling factor and release factor 3 action promotes TnaC-pep tidyl-tRNA dropoff and relieves ribosome stalling during tryptophan induction of tna operon expression in Escherichia coli. J. Bacteriol. 2007 189 3147-3155. 

T. Fujiwara, K. Ito, and Y. Nakamura. 2001. Functional mapping of ribosome-contact sites in the ribosome recycling factor A structural view from atRNA mimic 7 64-70. (PubMed)... 

The 70S ribosome is now unstable in the presence of a protein called ribosome recycling factors, as well as the initiation factors IF3 and IFl. Consequently, the 70S ribosome dissociates to SOS and 30S subunits and is ready for another round of translation. 

R 774 T. Yoshida, Structure and Function of Ribosome Recycling Factor (RRF) , Seibutsu Kogaku Kaishi, 2003, 81, 150... 

FIGURE 9 Termination of protein synthesis and ribosome recycling. In prokaryotes, RF1 hydrolyzes the newly synthesized protein at stop codons UAG and UAA, while RF2 recognizes stop codons UGA and UAA. The GTPase RFS stimulates release of either RF1 or RF2. In eukaryotes a single protein recognizes all stop codons. The final step of translation is dissociation of the inactive 70S complex, stimulated by the ribosome recycling factor (RRF). 

Fuiiwara, T-, Ito. K.. and akaimira,V 2001, Functional mapping of ribosome-contact. site.s in the ribo.some recycling factor. A structural view from a tRNA mimie. RNA 7 64 70. 

Ribosomes needed for translation in the PURE system are isolated from E. coli using sucrose-density gradient centrifugation. The protein factors necessary for translation in E. coli are recombinantly expressed as His-tagged fusions, and purified to homogeneity. These include the factors for initiation (IFl, IF2, and IF3), elongation (EF-G, FF-Tu, FF-Ts), peptide chain release (RFl and RF3), ribosome recycling (RRF), methionyl-tRNA transformylase (MTF) for formylation of the initial Met-tRNA, and the 20 aminoacyl-tRNA synthetases (ARSs) for transfer RNA (tRNA) recy-... 

As illustrated by the [PS / ] phenomenon the relative abundance of the eRFl/eRF3 complexes is an important determinant for the efficiency of translation termination. Similar effects have been obtained in depletion studies and with eRF3 and eRFl mutants that decrease their cellular concentration (Stansfield et al. 1996 Moskalenko et al. 2003 Chabelskaya et al. 2004 Salas-Marco and Bedwell 2004). Wild type yeast contains approximately 10- to 20-fold fewer termination factors compared to ribosomes (Didichenko et al. 1991 Stansfield et al. 1992). The threshold level of eRFl/eRF3 required to maintain viability is even lower. A 10-fold decrease reduces viability by only about 10% (Valouev et al. 2002) and a decrease in eRF3 of 99% does not affect viability of the cells significantly (Chabelskaya et al. 2004). As eRF3 is associated with polysomes or ribosomes, the mechanism of translation termination depends on efficient recycling (Didichenko et al. 1991 and compare below). 

In bacteria there are three release factor pvot ms RFl, RF2, and RF3. In response to the stop codons they bind (in various combinations) to the A site and activate peptidyl transferase that hydrolyzes the ester bond using water rather than an amino group as the acceptor of the peptide. This releases the chain from the tRNA in the P site and generates the free carboxy-terminus of the polypeptide. Also 16S rRNA plays a role recognizing stop codons. Once the peptide leaves the ribosome, the ribosome subunits dissociate and are recycled. 

Further work, summarized in Fig. 4, describes a cycle of dissociation and reassociation of the two ribosomal subunits (30S and 508) during protein biosynthesis, a cycle dependent on the recycling of the initiation factors discovered by Ochoa. 

By this stage, all initiation factors have been released and are available for recycling, although the exact steps at which factors are released from intermediate complexes are not known in every case. There is thus an initiation factor cycle within the ribosome cycle, and regulation of the activity of factors, particularly elF-2, is an important control mechanism in translation (see 8ection V.D.1). 

Kaempfer, R., and Kaufman, J., 1972, Translational control of hemoglobin synthesis by an initiation factor required for recycling of ribosomes and for their binding to messenger RNA, Proc. Natl. Acad. Sci. USA 69 3317. 

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