14.5
RNA Stability
Intact DNA strands can be found in fossils, while scientists sometimes struggle to keep RNA intact under laboratory conditions. The structural variati…
RNA is a mobile, relatively short-lived molecule that is much less structurally and chemically stable compared to DNA. In RNA, the five-carbon sugar ribose has a hydroxyl group at the second carbon, while deoxyribose has a single hydrogen. The hydrogen of the hydroxyl group is susceptible to being removed in basic solutions. When this occurs, the negatively charged oxygen that remains is capable of breaking the phosphate sugar backbone.
In addition, RNA is usually single stranded, making it less structurally stable than the double helix of DNA. RNA molecules are also much shorter than DNA molecules, so they're more vulnerable to degradation at their ends. External factors can also influence the stability of RNA. For example, specific exonucleases in the cytoplasm called RNases break down RNAs that are not actively being translated. Other proteins, known as RNA-binding proteins effect stability by recognizing and binding to specific RNA nucleotide sequences.
mRNA transcripts with AU-rich elements, usually repeats of AUUUA, in their three-prime untranslated regions, or three-prime UTRs, attract different classes of RNA-binding proteins with opposing roles. Some of these proteins enhance mRNA stability and increase protein translation while bounded to the three-prime UTR, while others destabilize the transcript so that it is degraded more quickly. Thus, the amount of time that an RNA molecule is available for translation is variable and dependent on multiple factors.
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Q1: Why is RNA less stable than DNA?
RNA is less stable than DNA due to structural differences. RNA contains a hydroxyl group on the second carbon of ribose sugar, which is susceptible to removal in basic solutions, breaking the phosphate-sugar backbone. Additionally, RNA is typically single-stranded, unlike DNA's double helix structure, making it more vulnerable to degradation. RNA molecules are also much shorter than DNA, increasing their susceptibility to end degradation.
Q2: What role do RNA-binding proteins play in mRNA stability?
RNA-binding proteins regulate mRNA stability by recognizing and binding to specific nucleotide sequences, particularly AU-rich elements containing AUUUA repeats in the three-prime UTR. Some RNA-binding proteins enhance mRNA stability and increase protein translation, while others destabilize transcripts for faster degradation. The number of AUUUA repeats determines which class of protein is recruited, controlling how long mRNA remains available for translation.
Q3: How do RNases affect RNA molecules in the cytoplasm?
RNases are exonucleases in the cytoplasm that break down RNA molecules not actively being translated. These enzymes degrade RNA from the ends, exploiting the structural vulnerability of single-stranded RNA. However, the five-prime cap and polyA tail protect eukaryotic mRNA from degradation until the cell no longer needs the transcript, providing temporary protection against RNase activity.
Q4: What is the relationship between RNA structure and its vulnerability to degradation?
RNA's single-stranded structure makes it inherently less stable than double-stranded DNA, though it allows greater flexibility and weak internal bonding. The hydroxyl group on ribose sugar increases susceptibility to backbone breakage. Most cellular RNAs are relatively short compared to DNA, making them more vulnerable to degradation at their ends. These structural features allow cells to regulate RNA longevity and availability for translation.
Q5: How does methylation affect mRNA stability and function?
Methylation of adenosine residues, known as m6A modification, plays an important role in mRNA regulation. m6A appears to increase both mRNA translation and degradation rates, affecting how long transcripts remain available. This modification also influences stress responses, nuclear export, and mRNA maturation. Additionally, pseudouridine, a modified uracil residue, appears to play an important role in RNA regulation and stability.
Q6: Why is mRNA availability variable for protein translation?
mRNA availability for translation is variable because multiple factors control RNA stability. RNA-binding proteins with opposing roles bind to AU-rich elements in the three-prime UTR, either enhancing or destabilizing transcripts. RNases degrade unprotected RNA, while protective structures like the five-prime cap and polyA tail provide temporary defense. The combined effects of these regulatory mechanisms determine how long each mRNA molecule remains available for translation.
Q7: How do cells exploit RNA instability for gene regulation?
Cells regulate gene expression by controlling mRNA stability and longevity. More stable mRNAs remain available for translation longer than less stable transcripts, allowing differential protein production. RNA-binding proteins, RNases, and chemical modifications like m6A collectively determine transcript lifespan. This regulatory system enables cells to rapidly adjust protein levels by modulating which mRNAs persist and remain available for translation.