🦠 microbiology

RNA viruses that exploit self-cleaving ribozymes for translation initiation

This study reveals that diverse RNA viruses with linear genomes have co-opted conserved self-cleaving ribozymes, originally associated with circular viroid-like agents, to facilitate cap-independent translation initiation in fungi and plants.

Original authors: Lopez-Galiano, M. J., Rueda, O., Chiba, S., Forgia, M., Navarro, B., Cervera, A., Babaian, A., Di Serio, F., Turina, M., De la Pena, M.

Published 2026-02-18

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CC BY 4.0

Original authors: Lopez-Galiano, M. J., Rueda, O., Chiba, S., Forgia, M., Navarro, B., Cervera, A., Babaian, A., Di Serio, F., Turina, M., De la Pena, M.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine the world of viruses as a bustling city of tiny, self-replicating machines. For decades, scientists believed these machines followed a strict rulebook: if they had a "self-destruct" switch (a ribozyme), it was only there to help them copy their blueprints in a circle, like a train on a looped track. This was the old story.

But this new paper tells a thrilling plot twist: Some viruses have discovered a new superpower. They are taking that self-destruct switch and using it to start their engines.

Here is the story broken down into simple, everyday concepts:

1. The "Self-Cleaving Scissors" (The Ribozyme)

Think of a ribozyme as a tiny pair of molecular scissors built right into the virus's genetic code.

The Old Job: Historically, we knew these scissors were used by "circular" viruses (like viroids). Imagine a circular train track; the scissors cut the track to make it the right size so the train can keep looping. This is called "rolling circle replication."
The New Discovery: The researchers found these same scissors in linear viruses (viruses with straight, non-circular DNA/RNA lines). These viruses don't need to cut their track to loop it. So, why do they have scissors?

2. The "Cap-Independent" Engine Starter

Most viruses need a special "key" (called a cap) to start translating their genetic code into proteins. It's like needing a specific key to start a car.

The Problem: These linear viruses often lack that key.
The Solution: The researchers found that the viral "scissors" (ribozymes) act as a universal engine starter.
- The Analogy: Imagine a car that doesn't have a keyhole. Instead, it has a hidden button. If you press the button (which is the ribozyme cutting itself), the engine roars to life.
- The virus uses the act of cutting itself to create a new starting point for its instructions, allowing it to build proteins even without the standard "key."

3. The "Split Personality" Strategy

The paper reveals that these viruses are playing a clever game of balance.

The Mix: Inside an infected cell, the virus population is a mix of two types:
1. The Whole Package: Intact viruses ready to copy themselves (replicate).
2. The Cut Pieces: Viruses that the scissors have chopped. These pieces can't replicate anymore, but they are perfect for making proteins (translation).
The Strategy: It's like a bakery that bakes two types of bread. One type is the whole loaf, which they sell to make more dough (replication). The other type is sliced bread, which they sell immediately to customers for a quick snack (protein production). The virus sacrifices some of its "whole loaves" to ensure it can make enough "snacks" to keep the infection going.

4. The Evidence: From Fungi to Plants

The scientists didn't just guess; they tested this in the lab:

The Test: They took the "scissors" from a fungus virus and a plant virus and put them into a test tube and living cells.
The Result: When the scissors were working (cutting), the virus made proteins. When they broke the scissors (so they couldn't cut), the virus stopped making proteins, even though the rest of the virus was fine.
The Conclusion: The cutting action itself is the spark that ignites the engine.

5. Why This Matters

This discovery changes how we view the "RNA World."

Evolutionary Twist: It suggests that these ancient molecular scissors, which we thought were just for copying circular loops, have been "co-opted" (hijacked) by linear viruses to solve a different problem: how to start a conversation without a formal introduction.
The Big Picture: It shows that nature is incredibly creative. A tool designed for one job (cutting loops) can be repurposed for a completely different job (starting engines) in the microscopic world.

In a nutshell: These viruses have found a way to use their own self-destruct mechanism as a "start button" to turn on their protein-making factories, ensuring they can infect their hosts even without the standard tools other viruses use. It's a brilliant, chaotic, and efficient survival strategy.

1. Problem Statement

Small self-cleaving ribozymes (e.g., hammerhead, hepatitis delta, twister) were historically characterized as catalytic RNAs essential for the rolling-circle replication of circular RNA (circRNA) agents like viroids and viroid-like satellites. While recent discoveries have identified these motifs in diverse DNA genomes and linear RNA viruses, their specific functional roles in linear RNA viruses remained unclear. The central question addressed by this study is whether these ribozymes in linear viral genomes serve a traditional replication role or if they have been co-opted for novel functions, specifically regarding protein translation initiation.

2. Methodology

The researchers employed a multi-faceted approach combining bioinformatics, in vitro biochemistry, and in vivo functional assays:

Bioinformatic Screening:
- Used INFERNAL 1.1 with covariance models from the RFAM 14 database to screen 852,000 viral contigs (DNA and RNA) from GenBank.
- Identified conserved ribozyme motifs (hammerhead, deltavirus, twister) in linear RNA viruses from fungi and plants, specifically within the Chrysoviridae, Fusariviridae, Megabirnaviridae, Totiviridae, and Hypoviridae families.
- Developed new covariance models to account for novel ribozyme architectures found in these viruses.
In Vitro Kinetic Analysis:
- Synthesized ribozyme sequences from fungal and plant viruses (e.g., Verticillium dahliae chrysovirus, Brassica campestris chrysovirus) via T7 in vitro transcription.
- Measured self-cleavage rates ( $k_{obs}$ ) under co-transcriptional and post-transcriptional conditions at varying pH and Mg $^{2+}$ concentrations using PAGE and Sybr Gold staining.
In Vivo Characterization (RACE & Northern Blot):
- Performed 5' Rapid Amplification of cDNA Ends (RACE) on total RNA extracted from infected plants (Brassica oleracea) and fungi (Gnomoniopsis castanea, Pleospora typhicola).
- Used Northern blotting to detect subgenomic RNA fragments resulting from ribozyme cleavage.
Functional Translation Assays (Dual-Luciferase Reporter):
- Fungal System: Transformed Cryphonectria parasitica with bicistronic constructs containing the 5'-UTR of Helminthosporium victoriae virus 145S (HvV145S). The system measured cap-independent translation of a downstream Firefly luciferase (Fluc) relative to an upstream Renilla luciferase (Rluc).
- Plant System: Agroinfiltrated Nicotiana benthamiana with bicistronic constructs containing various isolated ribozyme motifs (hhrbz, dvrbz) between Rluc and Fluc genes to test if self-cleavage alone could induce translation.
- Mutational Analysis: Introduced point mutations (e.g., G159U in the catalytic core) and deletions to disrupt ribozyme structure or catalytic activity to determine the necessity of self-cleavage for translation.

3. Key Contributions

Discovery of Ribozymes in Linear Genomes: The study significantly expands the known distribution of small ribozymes, identifying them in the 5'-UTRs of diverse linear dsRNA and ssRNA viruses (fungi and plants), a context previously dominated by circRNA agents.
Novel Structural Variants: Identified specific variants of Type I hammerhead ribozymes (hhrbz) in viruses that lack the typical tertiary loop-loop interactions found in canonical hhrbzs, featuring either unusually long or absent helix II.
Functional Co-option: Demonstrated that these ribozymes are not merely processing elements for replication but are functionally co-opted to drive cap-independent translation initiation.
Mechanism of Action: Provided evidence that the self-cleaving activity itself is the critical factor enabling translation, rather than just the static RNA structure acting as an Internal Ribosome Entry Site (IRES).

4. Key Results

Prevalence:
- Chrysoviridae: 42% of alphachrysovirus sequences contain Type I hhrbz in their 5'-UTRs.
- Fusariviridae: 114 of 156 sequences contained hhrbz motifs.
- Other Families: Ribozymes (hhrbz, dvrbz, twister) were also found in Megabirnaviruses, Totiviruses, Hypoviruses, and Fusagraviruses.
- Tandem Pairs: Some Totiviruses contain tandem ribozyme pairs separated by 186–399 nt, potentially generating small circRNAs resembling Zetaviruses.
In Vitro Activity:
- Ribozymes with long helix II showed efficient self-cleavage ( $k_{obs} \approx 2-3 \text{ min}^{-1}$ ).
- Ribozymes with short or absent helix II showed low to very low cleavage rates ( $10^{-2}$ to $10^{-3} \text{ min}^{-1}$ ), suggesting a regulatory balance between cleavage and genome integrity.
In Vivo Evidence:
- RACE analysis confirmed the existence of both full-length and self-cleaved viral RNA species in infected hosts, proving that cleavage occurs in vivo.
- Northern blots confirmed the accumulation of subgenomic RNA fragments.
Translation Initiation:
- Fungal Assay: Deletion of the hhrbz motif or mutation of the catalytic G159 base in the HvV145S 5'-UTR completely abolished downstream Fluc expression. This indicates that the catalytic activity is essential for translation.
- Plant Assay: Isolated viral ribozymes (hhrbz and dvrbz) inserted into a bicistronic reporter induced detectable cap-independent translation in N. benthamiana, whereas a non-cleaving control did not.
- Correlation: The efficiency of translation initiation correlated with the specific ribozyme type and its cleavage efficiency (e.g., megabirnavirus hhrbz and deltavirus ribozymes performed better than some chrysovirus variants).

5. Significance

Paradigm Shift: The study challenges the traditional view that small ribozymes are exclusively involved in the replication of circular RNAs. It establishes a new evolutionary role where linear RNA viruses co-opt self-cleaving activity to regulate gene expression.
Translation Mechanism: It proposes a novel mechanism for cap-independent translation where the act of RNA cleavage (rather than just the folded structure) is required to expose the ribosome binding site or generate a specific RNA topology necessary for translation initiation.
Evolutionary Insight: The findings suggest a "modern RNA world" where mobile genetic elements blur the lines between viroids, satellites, and viruses, utilizing ancient catalytic motifs for complex regulatory functions in eukaryotic hosts.
Biotechnological Potential: These ribozymes could serve as new tools for synthetic biology to engineer cap-independent translation systems or regulate gene expression in plants and fungi.