Shine-Dalgarno ribosome binding site

EXAMPLE 9.30 During translation the ribosome moves along the mRNA and leaves the leader region (containing the Shine-Dalgarno ribosome binding site) empty. Is it possible for initiation of a new polypeptide chain to occnr before completion of the previous one ... 

Shine-Dalgarno sequence Binding site for the SOS ribosome on an mRNA for translation initiation, terminator Signal for RNA polymerase to stop transcription and release the DNA template and the nascent transcript. 

Shine-Dalgarno sequence (ribosome binding site) Initiation codon... 

Shine, J., and Dalgarno, L. (1974). The 3 -terminal sequence of Escherichia coli 16S ribosomal RNA complementarity to nonsense triplets and ribosome binding sites. Proceedings of the National Academy of Sciences of the United States of America 71, 1342-1346. 

Answer C. Incorporation of a Shine-Dalgarno sequence into the expression vector will promote ribosome binding to the translation start site on the mRNA produced by transcription of the cDNA insert. 

The first codon translated in all mRNAs is AUG which codes for methionine. This AUG is called the start codon or initiation codon. Naturally, other AUG codons also occur internally in an mRNA where they encode methionine residues internal to the protein. Two different tRNAs are used for these two types of AUG codon tRNAfMet is used for the initiation codon and is called the initiator tRNA whereas tRNAmMet is used for internal AUG codons. In prokaryotes the first amino acid of a new protein is /V-formylmethionine (abbreviated fMet). Hence the aminoacyl-tRNA used in initiation is fMet-tRNAfMet. It is essential that the correct AUG is used as the initiation codon since this sets the correct reading frame for translation (see Topic HI). A short sequence rich in purines (5 -AGGAGGU-3 ), called the Shine-Dalgarno sequence, lies 5 to the AUG initiation codon (Fig. 3) and is complementary to part of the 16S rRNA in the small ribo-somal subunit. Therefore this is the binding site for the 30S ribosomal subunit... 

By contrast, eucaryal mRNAs are translated after extensive modifications of the primary transcripts that yield mature (generally capped and polyadenylated) monocistronic mRNAs. Recognition of translation start sites does not rely upon Shine-Dalgarno recognition instead, the small ribosomal subunit (generally) binds to the capped 5 end of mRNA and scans its nucleotide sequence until the initiator AUG codon is encountered. The polypeptide chains are initiated by a non-formylated methionine and the initiation reactions are aided by as many as 8-10 protein factors, some of which possess ATPase activity and perform functions not encountered in bacteria, such as cap recognition and mRNA unwinding (for a detailed review see ref [4]). 

Formation of the translational initiation complex in bacteria involves ribosome interactions with a sequence in the mRNA that is upstream (50) of the AUG and called the Shine-Dalgarno sequence. The purine-rich Shine-Dalgarno sequence (AGGAGG) is complementary to a portion of the 16S rRNA. Step 1 involves the interaction of IF-1 and IF-3 with the 30S ribosome subunit to form a complex which is positioned precisely at the initiator codon (AUG). The addition of IF-2, GTP, and the f-Met-tRNAf-Met result in the formation of the preinitiation complex in step 2. In step 3, hydrolysis of GTP and the release of IF-2 and IF-1 allows for the binding of the 50S ribosome subunit which completes the assembly of the initiation complex. The P and A sites are also schematically... 

No. This is in marked contrast to the situation in bacterial mRNA that may contain multiple Shine-Dalgarno sequences that permit binding by ribosomes to initiate translation from multiple start sites. In eukaryotes, the mRNA that codes for protein is monocistronic and hence codes for a single protein. One explanation for why this comes about is that each eukaryotic mRNA molecule has only a single 5 cap that recruits ribosomes, and thus translation cannot be initiated from multiple start sites on the same mRNA molecule, although alternative start sites may be used. 

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