What is the Shine-Dalgarno Sequence?

The Shine-Dalgarno sequence is a specific sequence of nucleotides where the mRNA binds to the ribosomal unit during the translation initiation process. It was two Australian scientists, John Shine and Lynn Dalgarno, who found this sequence in 1973. It is commonly found in bacteria but is rarer in archebacteria.

What is the Shine-Dalgarno Sequence?

The Shine-Dalgarno sequence is a purine-rich region on the 5’ terminal of the mRNA, about 4-9 residues upstream of the initiation codon AUG, a codon for the amino acid methionine. It is this purine-rich sequence that helps the mRNA bind to the ribosomal unit, and this is known as the ribosome binding site.

Shine-Delgarno sequence is seen in mitochondria and chloroplasts, which is considered proof for the endosymbiont theory.

How does Shine-Dalgarno Sequence Work?

The formation of the initiation complex enables the proper binding of the initiation codon AUG with the anticodon UAC in the corresponding tRNA through complementary base pairing. At the same time, the Shine-Dalgarno sequence pairs with the anti-Shine-Dalgarno sequence (CCUCCU) as well.

This Shine-Dalgarno sequence distinguishes the interior initiation codons from other similar codons occurring in the interior of the polypeptide chains. It ensures that the ribosome starts translating the mRNA at the appropriate site, leading to the synthesis of the desired protein.

This sequence is followed by a consensus sequence of AGGAGG. This sequence is absent in eukaryotes since they have a different mechanism for the initiation process, which involves a cap at 5’ end and a scanning of the mRNA molecule. In E. coli, this sequence is AGGAGGU.

A polycistronic mRNA that has more than two messages that need to be translated, there will have a ribosomal-binding site or Shine-Dalgarno sequence present at the 5’ end of each message. This will help with the smooth translation of the messages or codons.

Importance of Shine-Dalgarno Sequence

Any change in the sequence due to mutation could cause incorrect translation or inefficiency in protein synthesis. Both these scenarios can alter the protein or reduce its potency.
Even a slight variation in the positioning of this sequence can alter the protein characteristics.
The spacing of this sequence can be engineered to improve the bacterial translation systems.
Shine-Dalgarno sequences are used in the construction of plasmids to enable proper translation of genes.
They are also useful for the construction of controlled protein production.

Functions of Shine-Dalgarno Sequence

Shine-Dalgarno sequence plays an important role in the initiation of protein translation of prokaryotes such as bacteria and archaea.
It is essential for the precise and efficient initiation of translation.
It helps with the proper alignment og the ribosome to the starting codon.
The presence of this sequence minimises errors in translation.
In addition, Shine-Dalgarno sequence helps with the proper reading throughout the translation.

Conclusion

In prokaryotes, it is not the 5’ end that is significant for the translation initiation, instead, it is the Shine-Dalgarno sequence, which is vital because there are likely to be several initiation codons and multiple messages to be translated.

References

Agarwal, P. V. |. V. (2004). Cell Biology, Genetics, Molecular Biology, Evolution, and Ecology: Evolution and Ecology. S. Chand Publishing.
Wen, D., Kuo, T., & Chou, H. D. (2020). The diversity of Shine-Dalgarno sequences sheds light on the evolution of translation initiation. RNA Biology, 18(11), 1489. https://doi.org/10.1080/15476286.2020.1861406
https://taylorandfrancis.com/knowledge/Medicine_and_healthcare/Medical_genetics/Shine-Dalgarno_sequence/
https://easybiologyclass.com/shine-dalgarno-sequence-sd-sequence-structure-and-function/