14.5
RNA-Stabilität
Mitunter findet man intakte DNA-Stränge in Fossilien, jedoch haben Wissenschaftler manchmal sogar im Labor Schwierigkeiten RNA intakt zu halten. Die s…
RNA ist ein mobiles, relativ kurzlebiges Molekül, das im Vergleich zu DNA strukturell und chemisch viel weniger stabil ist. In der RNA hat die Ribose mit fünf Kohlenstoffen eine Hydroxylgruppe am zweiten Kohlenstoff, während Desoxyribose ein einzelnes Wasserstoffatom hat. Der Wasserstoff der Hydroxylgruppe kann leicht in basischen Lösungen entfernt werden.
In diesem Fall kann der verbleibende negativ geladene Sauerstoff das Phosphat-Zucker-Gerüst aufbrechen. Darüber hinaus ist RNA in der Regel einzelsträngig, wodurch es strukturell weniger stabil ist als die Doppelhelix der DNA. RNA-Moleküle sind auch viel kürzer als DNA-Moleküle, sodass sie an ihren Enden anfälliger für Zersetzung sind.
Externe Faktoren können auch die Stabilität der RNA beeinflussen. Beispielsweise spalten spezifische Exonukleasen im Zytoplasma, die RNasen, RNAs auf, die nicht aktiv translatiert werden. Andere Proteine wie RNA-bindende Proteine haben Auswirkungen auf die Stabilität indem sie spezifische RNA-Nukleotidsequenzen erkennen und sich an diese binden.
mRNA-Transkripte mit AU-reichen Elementen, normalerweise Wiederholungen von AUUUA, in ihren nicht translatierten Drei-Strich-Regionen, oder Drei-Strich UTRs, ziehen verschiedene Klassen von RNA-bindenden Proteinen mit entgegengesetzten Rollen an. Einige dieser Proteine verbessern die mRNA-Stabilität, und erhöhen die Proteintranslation während sie an das Drei-Strich-UTR gebunden sind, während andere Proteine das Transkript destabilisieren, sodass es schneller degradiert. Somit ist die Dauer, in der ein RNA-Molekül für die Translation verfügbar ist, variabel und von mehreren Faktoren abhängig.
View the full transcript and gain access to JoVE Core videos
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.