Codon for methionine

In addition to codons for ammo acids there are start and stop codons Protein biosynthesis begins at a start codon and ends at a stop codon of mRNA The start codon IS the nucleotide triplet AUG which is also the codon for methionine The stop codons are UAA UAG and UGA... 

FIGURE 28 12 Translation of mRNA to an ammo acid sequence of a protein starts at an mRNA codon for methionine Nucleophilic acyl substitution transfers the N formylmethionme residue from Its tRNA to the ammo group of the next ammo acid (shown here as alanine) The process converts an ester to an amide... 

Section 28 12 The start codon for protein biosynthesis is AUG which is the same as the codon for methionine Thus all proteins initially have methionine as their N terminal ammo acid but lose it subsequent to their formation The reaction responsible for extending the protein chain is nucleophilic acyl substitution... 

Methionine Has Only One Codon Methionine is one of two amino acids with only one codon. How does the single codon for methionine specify both the initiating residue and interior Met residues of polypeptides synthesized by E. coli ... 

Complementary, antiparallel bindinc of the anticodon for methionyl-tRNA (CAU) to the mRNA codon for methionine (AUG). 

As in prokaryotes, a special initiator tRNA is required for initiation and is distinct from the tRNA that recognizes and binds to codons for methionine at internal positions in the mRNA. When charged with methionine ready to begin initiation, this is known as Met-tRNAimet. 

The protein synthetic machinery must select the appropriate starting points for mRNA reading and peptide bond formation. AUG is usually used as the starting codon, and essentially all proteins begin with a methionine. AUG is also the codon for methionine that occurs in the interior of a protein as well, so there must be a mechanism to distinguish between the two types of methionine codons. 

From Table 17.1. the codon for methionine is seen to be AUG. Methionine is one of two amino acids for which there is only one codon. 

This stands for chain initiation and indicates that the codon for methionine (AUG) defines the beginning of the translated portion of an mRNA i.e., methionine is the first amino acid to be incorporated into a polypeptide chain. Less frequently GUG, normally for valine, can function in place of AUG as the chain-initiation codon incorporating methionine (see Table 17.1). It should be noted that while the first amino acid of a newly synthesized polypeptide chain is always methionine (or valine), methionine and valine may also occur within a polypeptide chain. 

A surprise was encountered when the sequence of human mitochondrial DNA became known. Human mitochondria read UGA as a codon for tryptophan rather than as a stop signal (Table 5.5). Furthermore, AGA and AGG are read as stop signals rather than as codons for arginine, and AUA is read as a codon for methionine instead of isoleucine. 

A. Methionine, the amino add that initiates the synthesis of proteins, is subsequently cleaved from the protein. The only codon for methionine is AUG, which serves as the codon for methionine residues within a protein as well as the initiating residue. The methionyl-tRNA for initiation is formylated in bacterial cells and in mitochondria. 

A. AUG is the initiation codon for methionine. The subsequent bases, read sequentially in sets of three, would all produce phenylalanine, except the termination codon UAA. 

ATG or AUG. The codon for methionine the translation initiation codon. Usually, protein translation can only start at a methionine codon (although this codon may be found elsewhere within the protein sequence as well). In eukaryotic DNA, the sequence is ATG in RNA it is AUG. Usually, the first AUG in the mRNA is the point at which translation starts, and an open reading frame follows, i.e., the nucleotides taken three at a time will code for the amino acids of the protein, and a stop codon will be found only when the protein coding region is complete. 

The sequence AUG serves as both an initiation codon and the codon for methionine. Since methionine occurs within protein chains, some signal in the base sequence of the mRNA must identify particular AUG triplets as start codons. In eukaryotes, i nitiation usually begins at the AUG triplet nearest to the 5 terminus of the mRNA molecule, i.e., no particular signal is used (although sequences do play a role in the efficiency of initiation). Presumably, the ribosome and the mRNA interact at the 5 terminus and slide with respect to one another until an AUG is encountered. This AUG determines the reading frame. When a stop codon is encountered, the ribosome and the mRNA dissociate. Only a unique polypeptide is translated from a particular eukaryotic mRNA molecule. 

Synthesis of all polypeptide chains In prokaryotic and eukaryotic cells begins with the amino acid methionine. In most mRNAs, the start (initiator) codon specifying this amino-terminal methionine is AUG. In a few bacterial mRNAs, GUG is used as the initiator codon, and CUG occasionally is used as an initiator codon for methionine in eukaryotes. The three codons UAA, UGA, and UAG do not specify amino acids but constitute stop (termination) codons that mark the carboxyl terminus of polypeptide chains in almost all cells. The sequence of codons that runs from a specific... 

The AUG codon for methionine is the most common start codon, specifying the amino acid at the NH2-termlnus of a protein chain. Three codons (UAA, UAG, UGA) function as stop codons and specify no amino acids. 

Protein biosynthesis is generally initiated by the codon for methionine. Bacteria require iV-formyl methionine. However, studies on cells from higher forms of life indicate that the methionine suffers subsequent scission from the growing protein after incorporation of 15-20 amino acids. 

Figure 12.14. The RNA codons for mRNA read from DNA and are installed on the ribosome to be read by tRNA bringing the amino acids to the ribosome for peptide (protein) synthesis. Note that AUG is both the start codon (and thus requires the tRNA anticodon of UAC for the first amino acid, methionine [Met, M]) as well as the codon for methionine (Met, M) that might be found elsewhere, and that UAA, UGA, and UAG are stop codons. Note, too, that this 4 X 16 array, with 64 combinations, is redundant for the 20 amino acids, and thus some amino acids are specified in more than one way, while the start codon is unique. Information in this table was obtained from Nirenberg, M Leder, R Bernfield, M. Brimacombe, R. Trupin, I Rottman, F. O Neal, C. Nat. Acad. Sd. U.S., 1965,53,1161 and subsequent work.

The first codon of mRNA (the initiation codon) is always AUG, the codon for methionine. This means that the amino terminus of all newly synthesized proteins is methionine, although this may well be removed in post-translational modification of the protein (section 9.2.3.4). 

An initiator methionine tRNA forms a complex with the small ribosomal subunit, then together with a variety of initiation factors (enzymes and other proteins) binds to the initiator codon of mRNA, and finally to a large ribosomal subunit, to form the complete ribosome. The 5 cap of mRNA is important for this process, as it marks the position of the initiator codon. AUG is the only codon for methionine, and anywhere... 

Initiation of the translation is well known in E. coli. The first step, promoted by a proteic factor (F3 or B) is the formation of a complex between a 30 S ribosomal subparticle and the initiation site of a messenger RNA (AUG codon for methionine). One particular species of Met-tRNA , on which the NH2 group of methionine may be formylated after transfer-RNA acylation, associates to this complex if other factors (F2 and Fi, or C and A) and GTP are present (GTP is included in the resulting complex). An entity called complex I is thus formed, and is then completed to complex II by addition of a SOS ribosomal particle. In this complex II, formylmet-tRNA Ms bound to the A (acceptor) site on the ribosome. The last step in the initiation process, which is catalysed by the F2 factor and which involves GTP hydrolysis to GDP and Pi, is the translocation of the formyl-met-tRNA to the P (donor) site on the ribosome in this way the so-called complex III is formed. (Ilie A and P sites on the ribosome were defined by using the property of puromycin to only react with a peptidyl-tRNA if this entity is at the donor (P) site.) The precise role of the different initiation factors which are obtained from the ribosome wash is not yet completely established. 

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