Ribosomal binding site

A ribosomal binding site (RBS) is a sequence of nucleotides upstream of the start codon of an mRNA transcript that is responsible for the recruitment of a ribosome during the initiation of protein translation. Mostly, RBS refers to bacterial sequences, although internal ribosome entry sites or IRES have been described in mRNAs of eukaryotic cells or viruses that infect eukaryotes. Ribosome recruitment in eukaryotes is generally mediated by the 5' cap present on eukaryotic mRNAs.

Prokaryotes

The RBS in prokaryotes is a region upstream of the start codon. This region is called the Shine-Dalgarno (SD) sequence.[1] The SD sequence on a messenger RNA is rich in purines and has the consensus AGGAGG. The complementary sequence (CCUCCU), called the anti-Shine-Dargarno (ASD) is contained in the 3’ end of the 16S region of the smaller (30S) ribosomal subunit. Upon encountering the Shine-Dalgarno sequence, the ASD of the ribosome base-pairs with it, after which translation is initiated.[2][3]

Effect on translation initiation rate

Prokaryotic ribosomes begin translation of the mRNA transcript while DNA is still being transcribed. Thus translation and transcription are parallel processes. Bacterial mRNA are usually polycistronic and contain multiple ribosomal binding sites. Translation initiation is the most highly regulated step of protein synthesis in prokaryotes.[4]

The rate of translation depends on two factors:

The RBS sequence affects both of these factors.

Factors affecting rate of ribosome recruitment

The ribosomal protein S1 binds to adenine sequences upstream of the RBS. Increasing the concentration of adenine upstream of the RBS will increase the rate of ribosome recruitment.[4]

Factors affecting the efficiency of translation initiation

The level of complementarity of the mRNA SD sequence to the ribosomal ASD greatly affects the efficiency of translation initiation. Richer complementarity results in higher initiation efficiency.[5] It is worth noting that this only holds up to a certain point - having too rich of a complementarity is known to paradoxically decrease the rate of translation as the ribosome then happens to be bound too tightly to proceed downstream.[5]

The optimal distance between the RBS and the start codon is variable - it depends on the portion of the SD sequence encoded in the actual RBS and its distance to the start site of a consensus SD sequence. Optimal spacing increases the rate of translation initiation once a ribosome has been bound.[5] The composition of nucleotides in the spacer region itself was also found to affect the rate of translation initiation in one study.[6]

Heat shock proteins

Secondary structures formed by the RBS can affect the translational efficiency of mRNA, generally inhibiting translation. These secondary structures are formed by H-bonding of the mRNA base pairs and are sensitive to temperature. At a higher-than-usual temperature (~42 °C), the RBS secondary structure of heat shock proteins becomes undone thus allowing ribosomes to bind and initiate translation. This mechanism allows a cell to quickly respond to an increase in temperature.[4]

Eukaryotes

5' cap

Ribosome recruitment in eukaryotes happens when eukaryote initiation factors elF4F and poly(A) binding protein (PABP) recognize the 5' capped mRNA and recruit the 43S ribosome complex at that location.[7]

Translation initiation happens following recruitment of the ribosome, at the start codon (underlined) found within the Kozak consensus sequence ACCAUGG. Since the Kozak sequence itself is not involved in the recruitment of the ribosome, it is not considered a ribosomal binding site.[7][2]

Internal ribosome entry site (IRES)

Eukaryotic ribosomes are known to bind to transcripts in a mechanism unlike the one involving the 5' cap, at a sequence called the internal ribosome entry site. This process is not dependent on the full set of translation initiation factors (although this depends on the specific IRES) and is commonly found in the translation of viral mRNA.[8]

Gene annotation

The identification of RBS is used to determine the site of translation initiation an unannotated sequence. This is referred to as N-terminal prediction. This is especially useful when multiple start codons are situated around the potential start site of the protein.[9][10]

It is particularly difficult to identify RBS because they tend to be highly degenerated.[11] One approach to identifying RBS in E.coli is using neural networks.[12] Another approach is using the Gibbs sampling method.[9]

History

The Shine-Dalgarno sequence, of the prokaryotic RBS, was discovered by John Shine and Lynne Dalgarno in 1975.[13][1]

The Kozak consensus sequence was first identified by Marilyn Kozak in 1984[14] while she was in the Department of Biological Sciences at the University of Pittsburgh.[15]

See also

References

  1. 1 2 Shine, J.; Dalgarno, L. (1975-03-06). "Determinant of cistron specificity in bacterial ribosomes". Nature 254 (5495): 34–38. doi:10.1038/254034a0.
  2. 1 2 "Ribosomal Binding Site Sequence Requirements". www.thermofisher.com. Retrieved 2015-10-16.
  3. ↑ "Help:Ribosome Binding Site - parts.igem.org". parts.igem.org. Retrieved 2015-10-16.
  4. 1 2 3 Laursen, Brian Søgaard; Sørensen, Hans Peter; Mortensen, Kim Kusk; Sperling-Petersen, Hans Uffe (2005-03-01). "Initiation of Protein Synthesis in Bacteria". Microbiology and Molecular Biology Reviews 69 (1): 101–123. doi:10.1128/MMBR.69.1.101-123.2005. ISSN 1092-2172. PMC 1082788. PMID 15755955.
  5. 1 2 3 De Boer, Herman A.; Hui, Anna S. (1990-01-01). Enzymology, BT - Methods in, ed. [9] Sequences within ribosome binding site affecting messenger RNA translatability and method to direct ribosomes to single messenger RNA species. Gene Expression Technology 185. Academic Press. pp. 103–114.
  6. ↑ Stormo, Gary D.; Schneider, Thomas D.; Gold, Larry M. (1982-05-11). "Characterization of translational initiation sites in E. coli". Nucleic Acids Research 10 (9): 2971–2996. doi:10.1093/nar/10.9.2971. ISSN 0305-1048. PMID 7048258.
  7. 1 2 Hellen, Christopher U. T.; Sarnow, Peter (2001-07-01). "Internal ribosome entry sites in eukaryotic mRNA molecules". Genes & Development 15 (13): 1593–1612. doi:10.1101/gad.891101. ISSN 0890-9369. PMID 11445534.
  8. ↑ Pisarev, Andrey V.; Shirokikh, Nikolay E.; Hellen, Christopher U.T. "Translation initiation by factor-independent binding of eukaryotic ribosomes to internal ribosomal entry sites". Comptes Rendus Biologies 328 (7): 589–605. doi:10.1016/j.crvi.2005.02.004.
  9. 1 2 Hayes, William S.; Borodovsky, Mark (1998). "Deriving ribosomal binding site (RBS) statistical models from unannotated DNA sequences and the use of the RBS model for N-terminal prediction." (PDF). Pacific Symposium on Biocomputing 3: 279–290.
  10. ↑ Noguchi, Hideki; Taniguchi, Takeaki; Itoh, Takehiko (2008-12-01). "MetaGeneAnnotator: Detecting Species-Specific Patterns of Ribosomal Binding Site for Precise Gene Prediction in Anonymous Prokaryotic and Phage Genomes". DNA Research: An International Journal for Rapid Publication of Reports on Genes and Genomes 15 (6): 387–396. doi:10.1093/dnares/dsn027. ISSN 1340-2838. PMC 2608843. PMID 18940874.
  11. ↑ Oliveira, Márcio Ferreira da Silva; Mendes, Daniele Quintella; Ferrari, Luciana Itida; Vasconcelos, Ana Tereza Ribeiro. "Ribosome binding site recognition using neural networks". Genetics and Molecular Biology 27 (4): 644–650. doi:10.1590/S1415-47572004000400028. ISSN 1415-4757.
  12. ↑ Stormo, Gary D. (2000-01-01). "DNA binding sites: representation and discovery". Bioinformatics 16 (1): 16–23. doi:10.1093/bioinformatics/16.1.16. ISSN 1367-4803. PMID 10812473.
  13. ↑ "Prof John Shine — Garvan Institute of Medical Research". www.garvan.org.au. Retrieved 2015-11-10.
  14. ↑ Kozak, Marilyn (1984-01-25). "Compilation and analysis of sequences upstream from the translational start site in eukaryotic mRNAs". Nucleic Acids Research 12 (2): 857–872. doi:10.1093/nar/12.2.857. ISSN 0305-1048. PMID 6694911.
  15. ↑ "Research: Top 10 Women Scientists Of The '80s: Making A Difference | The Scientist Magazine®". The Scientist. Retrieved 2015-11-10.
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