Ribosome transfer between proteins

RFl and RF2 are compact proteins that in eukaryotes resemble a tRNA molecule. When bound to the ribosome, the proteins unfold to bridge the gap between the stop codon on the mRNA and the peptidyl transferase center on the SOS subunit. Although the precise mechanism of release is not known, the release factor may promote, assisted by the peptidyl transferase, a water molecule s attack on the ester linkage, freeing the polypeptide chain. The detached polypeptide leaves the ribosome. Transfer RNA and messenger RNA remain briefly attached to the 70S ribosome until the entire complex is dissociated in a GT F-dependent fashion in response to the binding of EF-G and another factor, called the ribosome release factor (RRF) (Figure 30.25). 

Mitochondria are unique organelles in that they contain their own DNA (mtDNA), which, in addition to ribosomal RN A (rRNA) and transfer RN A (tRNA)-coding sequences, also encodes 13 polypeptides which are components of complexes I, III, IV, and V (Anderson et al., 1981). This fact has important implications for both the genetics and the etiology of the respiratory chain disorders. Since mtDNA is maternally-inherited, a defect of a respiratory complex due to a mtDNA deletion would be expected to show a pattern of maternal transmission. However the situation is complicated by the fact that the majority of the polypeptide subunits of complexes I, III, IV, and V, and all subunits of complex II, are encoded by nuclear DNA. A defect in a nuclear-coded subunit of one of the respiratory complexes would be expected to show classic Mendelian inheritance. A further complication exists in that it is now established that some respiratory chain disorders result from defects of communication between nuclear and mitochondrial genomes (Zeviani et al., 1989). Since many mitochondrial proteins are synthesized in the cytosol and require a sophisticated system of posttranslational processing for transport and assembly, it is apparent that a diversity of genetic errors is to be expected. 

What is the importance of this enzyme family for the biogenesis problem These enzymes form the link between the protein world and the nucleic acid world . They catalyse the reaction between amino acids and transfer RNA molecules, which includes an activation step involving ATR The formation of the peptide bond, i.e., the actual polycondensation reaction, takes place at the ribosome and involves mRNA participation and process control via codon-anticodon interaction. 

Transfer (tRNA) and ribosomal RNA (rRNA) are transcribed and used in protein synthesising processes. Messenger RNA (mRNA) codes the amino-acid sequence of proteins and 95 per cent of the total DNA transcribed is used for this purpose. In prokaryotes a single mRNA molecule may code for a single polypetide or for two or more polypeptide chains. There is a triplet code for each amino-acid 300 ribonucleotides code for a 100 amino-acid sequence. Fig. 5.All shows the relationship between the nucleotide sequence on DNA and RNA and the amino-acid sequence of protein. 

Transfer RNAs (tRNAs) have two functions that link the RNA and protein information systems. First, they must accept a specific amino acid, one of 20. They do this with accuracy greater than 99.99 percent, even distinguishing between chemically similar structures. But tRNAs share functions as well. The translating ribosome must be able to insert any of the 20 amino acids at the correct position in the growing polypeptide chain, with roughly the same efficiency. Otherwise, the number of proteins that a cell could make would be severely limited. This means that all tRNAs must have common structural features that are recognized by the ribosome. 

Transfer RNA molecules (tRNAs), messenger RNA, and many proteins participate in protein synthesis along with ribosomes. The link between amino acids and nucleic acids is first made by enzymes called aminoacyl-tRNA synthetases. By specifically linking a particular amino acid to each tRNA, these enzymes implement the genetic code. This chapter focuses primarily on protein synthesis in prokaryotes because it illustrates many general principles and is relatively well understood. Some distinctive features of protein synthesis in eukaryotes also are presented. 

---