Codons components

Sachs, M. S., Wang, Z., Gaba, A., Fang, P., Belk, J., Ganesan, R., Amrani, N., and Jacobson, A. (2002). Toeprint analysis of the positioning of translation apparatus components at initiation and termination codons of fungal mRNAs. Methods 26, 105-114. 

The ribosome is both the site of protein synthesis and an active participant in the process. The eukaryotic ribosome is constructed from two subunits the smaller 40S subunit and the larger 60S subunit. Basically, the 40S subunit binds the mRNA and monitors the recognihon between the mRNA codon and tRNA anticodon. The 60S subunit has the binding sites for aminoacyl-tRNAs and catalyzes the formation of peptide bonds. Remarkably, the catalytic entity for peptide bond formahon in the 60S subunit is the RNA component, not the protein component. Therefore, the 60S subunit acts as a ribozyme. 

Recent studies have shown that proteins interacting with the release factors can modulate the efficiency of stop codon readthrough. Physical and functional interaction with the translation termination factors was demonstrated for different components of the translational machinery. 

Initiation of protein synthesis involves the assembly of the components of the translation system before peptide bond formation occurs. These components include the two ribosomal subunits, the mRNA to be translated, the aminoacyl-tRNA specified by the first codon in the message, GTP (which provides energy for the process), and initiation factors that facilitate the assembly of this initiation complex (see Figure 31.13). [Note In prokaryotes, three initiation factors are known (IF-1, IF-2, and IF-3), whereas in eukary- otes, there are at least ten (designated elF to indicate eukaryotic origin).] There are two mechanisms by which the ribosome recognizes the nucleotide sequence that initiates translation ... 

Initiation The components of the translation system are assembled, and mRNA associates with the small ribosomal subunit. The process requires initiation factors. In prokaryotes,a purine-rich region (the Shine-Dalgarno sequence) of the mRNA base-pairs with a complementary sequence on 16S rRNA, resulting in the positioning of the mRNA so that translation can begin. The 5 -cap on eukaryotic mRNA is used to position that structure on the ribosome. The initiation codon is 5 -AUG-3. ... 

The unusual amino acid selenocysteine (a derivative of cysteine in which the sulfur atom is replaced by a selenium atom) is an essential component in a small number of proteins. These proteins occur in prokaryotes and eukaryotes ranging from E. coli to humans. In all cases, selenocysteine is incorporated into protein during translation in response to the codon UGA. This codon usually serves as a termination codon but occasionally, in some required but unknown context of bases, is used to specify selenocysteine instead. 

BSEP also known as sister-P-glycoprotein (SPGP) was originally cloned from pig liver (185). BSEP is localized on the canalicular membrane of hepa-tocytes and is responsible for the secretion of bile salts across the canalicular membrane into bile. BSEP appears to be the predominant bile salt efflux system for hepatocytes, and is a critical component in the enterohepatic circulation of bile acids. A number of mutations in the transporter were found to the basis for progressive familial intrahepatic cholestasis type 2 (PFIC2) (186-188). Mutations found in PFIC2 patients include frameshifts, missense mutations, and premature termination codons. Most PFIC2 patients lack immunohistochemically detectable BSEP in their liver. Recently, seven... 

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