Methionine codon

MBD2b a truncated version of MBD2a (translation stars at a second methionine codon in MBD2a) that lacks the (GR)n domain was reported to have demethylase activity with direct removal of the methyl group expressed in somatic tissues but not ES cells... 

Simplicity argues that the genetic blueprint specifying amino acid sequences in proteins should consist of consecutive, nonoverlapping triplets. This assumption turned out to be correct, as is illustrated by the DNA sequence for a gene shown in Fig. 5-5. In addition to the codons that determine the sequence of amino acids in the protein, there are stop codons that tell the ribosomal machinery when to terminate the polypeptide chain. One methionine codon serves as an initiation codon that marks the beginning of a polypeptide sequence. One of the valine codons sometimes functions in the same way. 

For most polypeptide chains initiation begins with one of the three initiation codons, most commonly the methionine codon AUG. When properly placed in an mRNA chain, GUG may also serve as a bacterial initiation codon. In such cases, it codes for methionine rather than for valine. Occasionally UUG, AUU, ACG, and perhaps other codons can initiate translation 288/289 This is less frequent in eukaryotes than in bacteria. The sequence of bases preceding the initiation codon must also be important for recognition of the "start" signal. 

Explain how the protein synthesizing machinery is able to differentiate the initiation AUG codon from an internal AUG (methionine) codon in prokaryotes. How is this accomplished in eukaryotes ... 

As can be seen from the alignment shown in Fig. 2, the sequence given in the database for the Tetraodon 5-HT4 receptor (accession number CAF95370) is unusually short, beginning with a methionine in TM1. This ATG is unlikely to be the true start codon for this receptor because this results in part of TM1 being absent, but examination of the genomic sequence shows a TAG stop codon immediately upstream of the ATG methionine codon. Whether this is... 

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. 

The trp operon has no positive control system like cAMP-CAP, but it does have an additional transcriptional control mechanism that depends on the concentration of tryptophan. This involves the attenuator site, which resides within the leader (L) sequence. It consists of 14 adjacent codons beginning with a methionine codon (AUG) and ending with a termination codon (UGA) and, importantly, codons (UGG) for tryptophan at positions 10 and 11. When tryptophan is plentiful, the complete 14-residue polypeptide (leader polypeptide) is synthesized. When tryptophan is scarce, the ribosome stalls at the tandem UGG... 

We routinely clone chemokines using polymerase chain reaction (PCR). The small size of chemokine genes, the frequent addition of N-terminal or C-terminal tags, and the need to localize precisely the initiator methionine codon in bacteria expression systems are all most easily dealt with by amplifying the genes. In this section, it is assumed that the reader has a grasp of basic cloning techniques (18) and is familiar with simple PCR amplification (21). 

Initiation codons. The methionine codon AUG is the most common starting point for translation of a genetic message but GUG can also serve. In such cases it codes for methionine rather than valine. 

Initiation of a polypeptide chain begins with the amino add methionine (codon = AUG). 

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. 

Describe how the base pairing between the Shine-Dalgamo sequence and the 30S subunit provides a mechanism for distinguishing a start codon from a methionine codon. What is the eukaryotic version of this mechanism ... 

The methionine anticodon (UAC) on the tRNA base pairs with the methionine codon AUG in the mRNA sequence that signals the start of protein synthesis. 

The methionine codon AUG functions both to initiate a polypeptide chain and to direct methionine incorporation into internal positions in a protein. By what mechanisms are the AUG start codons selected in prokaryotes ... 

The abbreviations for the amino acids are standard. The first, second and third nucleotides for each codon are as indicated. The Methionine codon AUO and Valine codon GUO also stand for initiator codons. The codons UAG, UAA and UGA stand for termination signab. 

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