Post-transcriptional regulation

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Post-transcriptional regulation is the control of gene expression at the RNA level, therefore between the transcription and the translation of the gene. [1][2]

Mechanism[edit]

After being produced, the stability and distribution of the different transcripts is regulated (post-transcriptional regulation) by means of RNA binding protein (RBP) that control the various steps and rates of the transcripts: events such as alternative splicing, nuclear degradation (exosome), processing, nuclear export (three alternative pathways), sequestration in DCP2-bodies for storage or degradation, and ultimately translation. These proteins achieve these events thanks to a RNA recognition motif (RRM) that binds a specific sequence or secondary structure of the transcripts, typically at the 5’ and 3’ UTR of the transcript.

Modulating the capping, splicing, addition of a Poly(A) tail, the sequence-specific nuclear export rates and in several contexts sequestration of the RNA transcript occurs in eukaryotes but not in prokaryotes. This modulation is a result of a protein or transcript which in turn is regulated and may have an affinity for certain sequences.

  • Capping changes the five prime end of the mRNA to a three prime end by 5'-5' linkage, which protects the mRNA from 5' exonuclease, which degrades foreign RNA. The cap also helps in ribosomal binding.
  • Splicing removes the introns, noncoding regions that are transcribed into RNA, in order to make the mRNA able to create proteins. Cells do this by spliceosomes binding on either side of an intron, looping the intron into a circle and then cleaving it off. The two ends of the exons are then joined together.
  • Addition of poly(A) tail otherwise known as polyadenylation. That is, a stretch of RNA that is made solely of adenine bases is added to the 3' end, and acts as a buffer to the 3' exonuclease in order to increase the half life of mRNA. In addition, a long poly(A) tail can increase translation. Poly(A)-binding protein (PABP) binds to a long poly(A) tail and mediates the interaction between EIF4E and EIF4G which encourages the initiation of translation.
  • RNA editing is a process which results in sequence variation in the RNA molecule, and is catalyzed by enzymes. These enzymes include the Adenosine Deaminase Acting on RNA (ADAR) enzymes, which convert specific adenosine residues to inosine in an mRNA molecule by hydrolytic deamination. Three ADAR enzymes have been cloned, ADAR1, ADAR2 and ADAR3, although only the first two subtypes have been shown to have RNA editing activity. Many mRNAs are vulnerable to the effects of RNA editing, including the glutamate receptor subunits GluR2, GluR3, GluR4, GluR5 and GluR6 (which are components of the AMPA and kainate receptors), the serotonin2C receptor, the GABA-alpha3 receptor subunit, the tryptophan hydroxlase enzyme TPH2, the hepatitis delta virus and more than 16% of microRNAs. In addition to ADAR enzymes, CDAR enzymes exist and these convert cytosines in specific RNA molecules, to uracil. These enzymes are termed 'APOBEC' and have genetic loci at 22q13, a region close to the chromosomal deletion which occurs in velocardiofacial syndrome (22q11) and which is linked to psychosis. RNA editing is extensively studied in relation to infectious diseases, because the editing process alters viral function.
  • mRNA Stability can be manipulated in order to control its half-life, and the poly(A) tail has some effect on this stability, as previously stated. Stable mRNA can have a half life of up to a day or more which allows for the production of more protein product; unstable mRNA is used in regulation that must occur quickly.

Significance[edit]

A prokaryotic example: Salmonella enterica (a pathogenic γ-proteobacterium) can express two alternative porins depending on the external environment (gut or murky water), this system involves EnvZ (osomotic sensor) which activates OmpR (transcription factor) which can bind to a high affinity promoter even at low concentrations and the low affinity promoter only at high concentrations (by definition): when the concentration of this transcription factor is high it activates OmpC and micF and inhibits OmpF, OmpF is further inhibited post-transcriptionally by micF RNA which binds to the OmpF transcript[3]

This area of study has recently gained more importance due to the increasing evidence that post-transcriptional regulation plays a larger role than previously expected. Even though protein with DNA binding domains are more abundant than protein with RNA binding domains, a recent study by Cheadle et al. (2005) showed that during T-cell activation 55% of significant changes at the steady-state level had no corresponding changes at the transcriptional level, meaning they were a result of stability regulation alone.[4]

Furthermore RNA found in the nucleus is more complex than that found in the cytoplasm: more than 95% (bases) of the RNA synthesized by RNA polymerase II never reaches the cytoplasm. The main reason for this is due to the removal of introns which account for 80% of the total bases.[5] Some studies have shown that even after processing the levels of mRNA between the cytoplasm and the nucleus differ greatly.[6]

Developmental biology is a good source of models of regulation, but due to the technical difficulties it was easier to determine the transcription factor cascades than regulation at the RNA level. In fact several key genes such as nanos are known to bind RNA but often their targets are unknown.[7] Although RNA binding proteins may regulate post transcriptionally large amount of the transcriptome, the targeting of a single gene is of interest to the scientific community for medical reasons, this is RNA interference and microRNAs which are both examples of posttranscriptional regulation, which regulate the destruction of RNA and change the chromatin structure. To study post-transcriptional regulation several techniques are used, such as RIP-Chip (RNA immunoprecipitation on chip).[8]

See also[edit]

References[edit]

  1. ^ Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, Peter Walter (2007). Molecular Biology of the Cell (Fifth ed.). Garland Science. pp. 1268 pages. ISBN 0-8153-4105-9. 
  2. ^ Weaver, Robert J. (2007). "Part V: Post-transcriptional events". Molecular Biology. Boston: McGraw Hill Higher Education. ISBN 0-07-110216-7. 
  3. ^ Mims C, Nash A, Stephen J. Mims' pathogenesis of infectious disease. 2001. 5th. Academic press. ISBN 0-12-498264-6
  4. ^ Cheadle C, Fan J, Cho-Chung YS, Werner T, Ray J, Do L, Gorospe M, Becker KG (2005). "Control of gene expression during T cell activation: alternate regulation of mRNA transcription and mRNA stability". BMC Genomics 6: 75. doi:10.1186/1471-2164-6-75. PMC 1156890. PMID 15907206. 
  5. ^ Jackson DA, Pombo A, Iborra F (2000). "The balance sheet for transcription: an analysis of nuclear RNA metabolism in mammalian cells". FASEB J. 14 (2): 242–54. PMID 10657981. 
  6. ^ Schwanekamp JA, Sartor MA, Karyala S, Halbleib D, Medvedovic M, Tomlinson CR (2006). "Genome-wide analyses show that nuclear and cytoplasmic RNA levels are differentially affected by dioxin". Biochim. Biophys. Acta 1759 (8–9): 388–402. doi:10.1016/j.bbaexp.2006.07.005. PMID 16962184. 
  7. ^ Scott F. Gilbert. Developmental Biology. Sinauer, 2003. ISBN 0-87893-258-5.
  8. ^ Keene JD, Komisarow JM, Friedersdorf MB (2006). "RIP-Chip: the isolation and identification of mRNAs, microRNAs and protein components of ribonucleoprotein complexes from cell extracts". Nat Protoc 1 (1): 302–7. doi:10.1038/nprot.2006.47. PMID 17406249.