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Chapter 13: Problem 22

Present an overview of various forms of posttranscriptional processing in eukaryotes. For each, provide an example.

Short Answer

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Question: Provide an overview of various forms of posttranscriptional processing in eukaryotes and their roles in gene expression. Answer: Posttranscriptional processing is a series of modifications that occur after the transcription of a gene into a pre-mRNA molecule in eukaryotic cells. These modifications generate a mature mRNA that is ready for translation and protects the mRNA from degradation. Some common posttranscriptional modifications include: 1. 5' capping - This adds a 7-methylguanosine cap to the 5' end of the mRNA molecule, protecting it from degradation, assisting in nuclear export, and helping recruit translation machinery. 2. 3' polyadenylation - This adds a poly(A) tail to the 3' end of an mRNA molecule, enhancing mRNA stability, nuclear export, and translation efficiency. 3. RNA splicing - This process removes non-coding introns from the mRNA and joins coding exons together to create a continuous reading frame for protein synthesis. 4. Alternative splicing - This allows a single gene to produce multiple mRNA isoforms through combinatorial use of exons, so that distinct protein isoforms can be generated for various cellular processes.

Step by step solution

01

Introduction to posttranscriptional processing

Posttranscriptional processing refers to a series of events that occur in eukaryotic cells after the transcription of a gene into a molecule of precursor messenger RNA (pre-mRNA). These events modify the pre-mRNA molecule to generate a mature mRNA that is ready for translation and protects the mRNA from degradation.
02

5' capping

5' capping is a posttranscriptional modification that adds a unique structure called a 7-methylguanosine cap (m7G) to the 5' end of the mRNA molecule. This cap is important for several reasons: it protects the mRNA from degradation by exonucleases; it assists in nuclear export of the mRNA; and it helps recruit the translation machinery during protein synthesis. Example: All eukaryotic mRNAs undergo 5' capping as part of their normal maturation process.
03

3' polyadenylation

3' polyadenylation is the process of adding a long chain of adenine nucleotides (a poly(A) tail) to the 3' end of an mRNA molecule. This process is essential for mRNA stability, nuclear export, and translation efficiency. The poly(A) tail is recognized by poly(A) binding proteins, which play important roles in these processes. Example: Most eukaryotic mRNAs, except for histone mRNAs, have a 3' poly(A) tail added to them during posttranscriptional processing.
04

RNA splicing

RNA splicing is the process of removing non-coding sequences (introns) from the mRNA and joining the coding sequences (exons) together to create a continuous reading frame for protein synthesis. This process is performed by a large protein-RNA complex called the spliceosome, which recognizes specific sequence elements at intron-exon boundaries and catalyses the splicing reaction. Example: The human 尾-globin pre-mRNA undergoes splicing to remove two introns and generate the mature mRNA that is subsequently translated into one of the protein subunits of hemoglobin.
05

Alternative splicing

Alternative splicing is a process that allows a single gene to produce multiple mRNA isoforms by combining exons in different ways during the splicing process. This greatly increases the proteomic complexity of eukaryotes by enabling the production of different protein isoforms from a single gene. Example: The human gene encoding the protein tropomyosin (TPM) undergoes alternative splicing to produce multiple mRNA isoforms, which are translated into distinct protein isoforms that have different roles in muscle contraction and other cellular processes.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

5' Capping
One of the first steps in posttranscriptional modification is the addition of the 5' cap to the nascent mRNA transcript. This cap is composed of a 7-methylguanosine molecule attached via a triphosphate bridge to the 5' end of the mRNA. The process of capping occurs shortly after transcription initiation and plays several crucial roles:
  • Protection: The 5' cap protects the mRNA from degradation by enzymatic activity, specifically by exonucleases that target the end of RNA molecules.
  • Nuclear Export: The cap structure is recognized by a set of proteins that assist in transporting the mature mRNA out of the nucleus into the cytoplasm, where translation occurs.
  • Translation Initiation: During protein synthesis, the 5' cap is essential for the ribosome to recognize the mRNA and begin translating it into a protein.
All mRNA molecules in eukaryotic cells, apart from the exceptions like rRNA and tRNA, undergo this 5' capping process as they mature.
3' Polyadenylation
Another key posttranscriptional modification is 3' polyadenylation. This involves adding a tail of adenine nucleotides to the 3' end of the mRNA transcript, known as the poly(A) tail. This modification is crucial for:
  • Maturation: Stabilizing the mRNA molecule and preventing its degradation by cellular enzymes.
  • Nuclear Export: Similar to the 5' cap, the poly(A) tail aids in the transport of the mRNA from the nucleus to the cytoplasm.
  • Translation Efficiency: Enhances the translation of mRNA by facilitating its recognition and engagement by the ribosome.
Most eukaryotic mRNAs receive a poly(A) tail during posttranscriptional processing, with histone mRNAs being a notable exception.
RNA Splicing
RNA splicing is an intricate process where non-coding regions called introns are removed from the pre-mRNA, leaving only the coding regions known as exons. This process is carried out by a dynamic complex known as the spliceosome, a molecular machine composed of proteins and small nuclear RNAs (snRNAs). Splicing ensures that:
  • The correct coding sequences are joined together to form a continuous open reading frame for translation.
  • Potentially deleterious sequences or non-coding RNA are removed prior to mRNA translation.
A classic example is the human 尾-globin gene, which has introns that need to be spliced out before producing the functional beta globin protein, an essential component of hemoglobin.
Alternative Splicing
Alternative splicing is a sophisticated mechanism that allows a single gene to yield multiple protein variants. This is achieved by selectively including or excluding certain exons during the splicing process, which can generate different mRNA isoforms from the same pre-mRNA. The benefits are:
  • Increased Proteomic Diversity: From a single gene, multiple protein forms can be produced, each potentially having distinct functions or regulatory properties in the cell.
  • Tissue-Specific Expression: Alternative splicing can lead to the production of proteins that are specific to certain tissues, enhancing organismal complexity and adaptability.
The TPM gene, encoding tropomyosin, exemplifies alternative splicing, allowing different proteins to be expressed in muscle versus non-muscle cells, adapting functionality to the specific protein needs of each tissue.

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Most popular questions from this chapter

M. Klemke et al. (2001) discovered an interesting coding phenomenon in which an exon within a neurologic hormone receptor gene in mammals appears to produce two different protein entities (XL \(\alpha\) s, ALEX). Following is the DNA sequence of the exon's \(5^{\prime}\) end derived from a rat. The lowercase letters represent the initial coding portion for the XL \(\alpha\)s protein, and the uppercase letters indicate the portion where the ALEX entity is initiated. (For simplicity, and to correspond with the RNA coding dictionary, it is customary to represent the noncoding, nontemplate strand of the DNA segment.) \(5^{\prime}-\) gtcccaaccatgcccaccgatcttccgcctgcttctgaagATGCGGGCCCAG (a) Convert the noncoding DNA sequence to the coding RNA sequence. (b) Locate the initiator codon within the XL \(\alpha\) segment. (c) Locate the initiator codon within the ALEX segment. Are the two initiator codons in frame? (d) Provide the amino acid sequence for each coding sequence. In the region of overlap, are the two amino acid sequences the same? (e) Are there any evolutionary advantages to having the same DNA sequence code for two protein products? Are there any disadvantages?

The concept of consensus sequences of DNA was defined in this chapter as sequences that are similar (homologous) in different genes of the same organism or in genes of different organisms. Examples were the Pribnow box and the -35 region in prokaryotes and the TATA-box region in eukaryotes. One study found that among 73 isolates from the virus HIV-Type \(1 \mathrm{C}\) (a major contributor to the AIDS epidemic), a GGGNNNNNCC consensus sequence exists (where \(\mathrm{N}\) equals any nitrogenous base) in the promoter-enhancer region of the \(\mathrm{NF}-\kappa \mathrm{B}\) transcription factor, a cis- acting element that is critical for initiating HIV transcription in human macrophages (Novitsky et al., 2002 ). The authors contend that finding this and other conserved sequences may be of value in designing an AIDS vaccine. What advantages would knowing these consensus sequences confer? Are there disadvantages as a vaccine is designed?

One form of posttranscriptional modification of most eukaryotic RNA transcripts is the addition of a poly-A sequence at the \(3^{\prime}\) end. The absence of a poly-A sequence leads to rapid degradation of the transcript. Poly-A sequences of various lengths are also added to many prokaryotic RNA transcripts where, instead of promoting stability, they enhance degradation. In both cases, RNA secondary structures, stabilizing proteins, or degrading enzymes interact with poly-A sequences. Considering the activities of RNAs, what might be general functions of \(3^{\prime}\) -polyadenylation?

A glycine residue is in position 210 of the tryptophan synthetase enzyme of wild-type \(E .\) coli. If the codon specifying glycine is GGA, how many single- base substitutions will result in an amino acid substitution at position \(210 ?\) What are they? How many will result if the wild-type codon is GGU?

The mRNA formed from the repeating tetranucleotide UUAC incorporates only three amino acids, but the use of UAUC incorporates four amino acids. Why?
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