JF - BBA Section Nucleic Acids And Protein Synthesis

McElroy SP, Nomura T, Torrie LS, et al. (2013) A lack of premature termination codon read‐through efficacy of PTC124 (Ataluren) in a diverse array of reporter assays. PLoS Biology 11: e1001593.

Protein Synthesis -Translation and Regulation

Eukaryotes use a surveillance mechanism to identify mRNAs with mutations (typically premature termination codons) or processing errors (such as incorrect splicing). Once detected, the aberrant messages are degraded to prevent the synthesis of truncated proteins. Recent evidence suggests that these mRNAs must be at least partially translated to determine whether a stop codon is in its proper context. In mammalian systems, the translating ribosome is proposed to measure the distance between the final splicing junction and the termination signal—if they are within 50 nucleotides of one another, termination is allowed to proceed. If they are further apart, either because of a misplaced stop codon within the mRNA reading frame or a splicing error, the mRNA is targeted for rapid degradation. How the ribosome recognizes this distance is not yet known. This mRNA surveillance is also called nonsense-mediated decay because the majority of mutational errors result in premature termination codons.


leads to premature termination …

Despite the types of translational errors described above, mRNA-directed protein synthesis is remarkably accurate. How is it that the ribosome and translational factors are able to achieve such faithful transmission of genetic information? One way to describe the specificity of cognate over noncognate AA-tRNAs is termed the "kinetic proofreading" mechanism. One can imagine that selection of an EF-Tu:GTP:AA-tRNA ternary complex by the ribosome during elongation can be considered a "scanning" step. Depending on the codon-anticodon interaction, the AA-tRNA will either bind irreversibly in the ri-bosomal A-site (in the case of the cognate AA-tRNA), or dissociate from the ribosome before or after GTP hydrolysis (noncognate AA-tRNA). In this model, the rate of EF-Tu-triggered GTP hydrolysis is the same for cognate and noncognate substrates. However, because the cognate AA-tRNA spends more time in complex with the ribo-some, GTP hydrolysis is likely to occur prior to dissociation of the tRNA, promoting tight binding of this tRNA in the A-site prior to peptide bond formation.


Premature termination of SMARCB1 translation may be followed by ..

Frameshift mutations occur because of a nucleotide insertion or deletion in a protein’s gene. Errors in reading frame also occur when the ribosome "slips" along the mRNA in such a way that the sequence is not read in triplets corresponding to the codons of the message, but the ribosome moves two or four nucleotides instead. These "slips" are — 1or +1 frameshifts, respectively. Fortunately the ribosome usually encounters a stop codon shortly after the frameshift; this minimizes the effect of the mutation. (Although frameshifting can be the unintended result of translational inaccuracy, "programmed" frameshifts also take place in special situations. These produce alternate polypeptides from a single mRNA.) Premature termination of a protein can occur when a nucleotide substitution produces a stop codon in the middle of the gene sequence. The peptidyl-tRNA may also dissociate prematurely from the ribosome before the stop codon is reached.

in order to inhibit protein synthesis

Insertion of selenocysteine requires several adaptations of the translational machinery. First, tRNASec is amino-acylated with serine by seryl-tRNA synthetase. The ser-ine attached to this misacylated species is then converted to selenocysteine by the enzyme selenocysteine synthase, which uses selenophosphate as a donor. Elongation factor Tu does not bind Sec-tRNASec as it does other elongator AA-tRNAs; instead, a unique protein SELB transports Sec-tRNASec to the ribosomal A-site. SELB is specific for Sec-tRNASec, rejecting other AA-tRNAs including Ser-tRNASec. The final novel feature of selenocysteine insertion is the mechanism of mRNA recognition. The UGA triplet can be used in the same organism as either a selenocysteine or a stop codon. The sequence context determines its recognition as a selenocysteine codon. The ternary complex SELB:GTP:Sec-tRNASec recognizes a stem-loop structure immediately 3′ (downstream) from a UGA codon that is read as selenocysteine. This structural feature of the mRNA is specifically bound by the carboxyl-terminal portion of SELB, while other regions of the protein are highly homologous to EF-Tu as expected. Insertion of selenocysteine into polypeptides therefore requires formation of a quaternary SELB:GTP:Sec-tRNASec:mRNA complex. This is in contrast to all other elongation steps in protein synthesis, which proceed through ternary complexes.

Protein synthesis (translation) - SlideShare

Bidou L, Hatin I, Perez N, et al. (2004) Premature stop codons involved in muscular dystrophies show a broad spectrum of readthrough efficiencies in response to gentamicin treatment. Gene Therapy 11: 619–627.