While the sprawls of London and Paris take center stage in the novel “A Tale of Two Cities,” we now seek to narrate an alternative epigenetic “tail” that tells all about two nucleotides! In this hefty tome, uridine and guanosine play the starring roles in a story passed down through generations and generations of worms; but just what the Dickens are we talking about?!
This age-old story from the lab of Scott Kennedy (Harvard Medical School, Boston, MA, USA) begins with the study of RDE-3, a ribonucleotidyltransferase involved in genome protection via transposon silencing and RNA interference (RNAi) in the model nematode worm C. elegans. These well-read researchers knew that RDE-3 could add two nucleotides, uridine (U) and guanosine (G), to the 3′ end of RNAs to form a poly (p)UG ribonucleotide tail, and now, their most recent research has revealed a captivating “Oliver Twist” to this story.
Fascinatingly, this eloquent ensemble of epigeneticists has now established that pUG tails turn RNAs into templates for the synthesis of small interfering RNAs (siRNAs) that can mediate transgenerational epigenetic inheritance in worms.
We all have “Great Expectations” for this tale, so let’s hear all about this epic new story from Shukla and colleagues:
- pUG PCR, which comprises reverse transcription of total C. elegans RNA using an adenosine-cytidine-based oligo followed by nested PCR, confirmed that RDE-3 adds pUG tails to mRNAs targeted for silencing by RNAi
- Importantly, RDE-3 also adds pUG tails to RNAs derived from potentially genome-damaging germline-expressed DNA transposons
- Instead of driving mRNA degradation, as perhaps expected, the injection of artificial pUG RNAs into worms demonstrated that pUG tails convert RNAs into “gene silencing agents”
- The switch from normal function to gene silencing requires more than eight 3′ UG repeats appended to more than 50 nucleotides of sense RNA
- The identification of proteins that bind to UG repeats established that RNA-dependent RNA polymerases bind to pUG tails and use the associated RNA as a template for the synthesis of small interfering (si)RNAs
- This process occurs in germline condensates termed Mutator foci that coordinate the entire process
- RNAi-triggered gene silencing can be inherited for multiple generations in C. elegans and serves as a prime example of transgenerational epigenetic inheritance
- The propagation of silencing by injected pUG-RNAs occurs over multiple generations before halting, perhaps as a function of the observed shortening of pUG-modified RNA length over time
- Transgenerational epigenetic inheritance mediated by pUG RNAs may occur in the absence of initiating RNA triggers via feed-forward amplification cycles of pUG RNA-templated siRNA synthesis coupled with siRNA-directed pUG RNA biogenesis
- This mechanism may enable worms to provide ongoing protection to their progeny against the activity of germline-expressed DNA transposons
These articulate authors next hope to explore the relevance of this mechanism in additional biological processes that require long-term memories of past expression states and to discover if the addition of polyribonucleotide tails to RNAs, and any associated alteration in function, occurs in other species.