What is RuBisCO and what is its typical role?

RuBisCO, or ribulose-1,5-bisphosphate carboxylase/oxygenase, is an enzyme that plays a crucial role in photosynthesis. It catalyzes the first step of the Calvin-Benson cycle, where carbon dioxide is captured and converted into organic compounds. In plants and algae that perform photosynthesis, RuBisCO is essential for converting light energy into chemical energy by fixing carbon dioxide.

How is Euglena longa's RuBisCO different from what is typically observed, and why is this significant?

In Euglena longa, a non-photosynthetic alga, the RuBisCO protein sequence, particularly the RBCL subunit, shows significant divergence from its photosynthetic counterparts, like Euglena gracilis. Despite not photosynthesizing, E. longa retains the rbcL gene, encoding the large subunit of RuBisCO. This divergence suggests that RuBisCO's function in E. longa may have shifted following the loss of photosynthesis. The presence of the gene and its altered form in a non-photosynthetic organism challenges our understanding of RuBisCO's traditional role and hints at a potential new function, or evolutionary degradation.

What evidence suggests RuBisCO might have a different role in Euglena longa than in photosynthetic organisms?

Several pieces of evidence point to a different role for RuBisCO in Euglena longa. First, the RBCL protein sequence has diverged from its photosynthetic counterparts. Second, while both RBCL and RBCS subunits are produced, they are present in low abundance compared to photosynthetic species like Euglena gracilis. Third, the RBCS monomers are undetectable in E. longa, indicating inefficient processing of the precursor polyprotein. Finally, post-transcriptional regulation, with rapid degradation of RBCS upon blocking cytoplasmic translation, further supports a functional difference.

Explain the role of the plastid and its relationship to photosynthesis and RuBisCO.

The plastid is an organelle found in plant cells and algae. It's the site of photosynthesis, the process where light energy is converted into chemical energy. Photosynthesis relies on the coordinated expression of genes from both the nucleus and the plastid itself. RuBisCO is a key player in this process, catalyzing the initial step of the Calvin-Benson cycle within the plastid. This cycle is crucial for fixing carbon dioxide and transforming it into organic compounds. The presence of RuBisCO is usually a clear indicator of the plastid's involvement in photosynthesis, but the study of Euglena longa challenges this understanding.

What are the potential implications of RuBisCO's presence in Euglena longa, and what questions remain for future research?

The presence of RuBisCO in the non-photosynthetic alga Euglena longa raises several intriguing possibilities. It could suggest that RuBisCO is being repurposed for a different, yet unknown, function in this organism. Alternatively, the findings may point to evolutionary degradation of the enzyme. Future research needs to clarify the specific role, if any, that RuBisCO plays in E. longa and other non-photosynthetic organisms. This includes investigating the precise function of the diverged RuBisCO protein, understanding the regulatory mechanisms controlling its expression, and determining the evolutionary history of RuBisCO in this and related species. This research could revolutionize our understanding of metabolic adaptation and evolution in algae.

Surreal illustration of deconstructed RuBisCO enzyme inside Euglena Longa cell, symbolizing declining function.

The Curious Case of RuBisCO: Why This Enzyme Still Matters in a World Without Photosynthesis

Nico Varela in Science & Nature September 2025 • 5 min read.

"Unraveling the mysteries of RuBisCO in Euglena longa: A journey through transcriptomic analysis and the unexpected role of a photosynthetic relic in a non-photosynthetic alga."

In the vast and intricate world of biology, the plastid stands out as a remarkable organelle. Found within plant cells and algae, it's best known for hosting photosynthesis, the life-sustaining process that converts light energy into chemical energy. This process relies heavily on the coordinated expression of genes from both the nucleus and the plastid itself. Among the key players in this elaborate dance is ribulose-1,5-bisphosphate carboxylase/oxygenase, more commonly known as RuBisCO.

RuBisCO holds the distinction of catalyzing the very first step of the Calvin-Benson cycle, a critical pathway in which carbon dioxide is captured and transformed into organic compounds. While RuBisCO is traditionally associated with photosynthesis, recent research has uncovered its presence in unexpected places, challenging our long-held assumptions about its sole purpose. One such case involves Euglena longa, a non-photosynthetic alga that, despite its inability to perform photosynthesis, retains the gene for RuBisCO's large subunit.

This discovery has ignited a spark of curiosity, prompting scientists to delve deeper into the enigmatic role of RuBisCO in E. longa. By examining its transcriptomic analysis and regulation of complex formation, researchers are beginning to unravel the secrets behind this photosynthetic relic and its potential implications for our understanding of algal evolution and metabolic adaptation.

RuBisCO in a Non-Photosynthetic World: What Does It Do?

Surreal illustration of deconstructed RuBisCO enzyme inside Euglena Longa cell, symbolizing declining function.

Euglena longa, a close relative of the photosynthetic model alga Euglena gracilis, presents a biological puzzle. While E. gracilis thrives through photosynthesis, E. longa has abandoned this process, possessing a non-photosynthetic plastid. Intriguingly, the genome of E. longa retains the rbcL gene, which encodes the large subunit of RuBisCO, the enzyme pivotal for carbon fixation in photosynthesis. This retention begs the question: What role does RuBisCO play in an organism that doesn't photosynthesize?

Recent research illuminates the story, revealing that the RuBisCO protein sequence in E. longa is significantly different from its photosynthetic counterparts. This divergence suggests a potential shift in function following the loss of photosynthesis. Like E. gracilis, E. longa possesses a nuclear gene encoding the small subunit of RuBisCO (RBCS) as a precursor polyprotein with multiple RBCS repeats, one of which is highly divergent. These findings indicate that E. longa might be repurposing RuBisCO for a different role.

Sequence Divergence: The RBCL protein in E. longa shows significant divergence, hinting at a functional shift.
RBCS Polyprotein: Similar to E. gracilis, E. longa encodes RBCS as a precursor polyprotein with multiple repeats.
Low Abundance: Both RBCL and RBCS proteins are synthesized in E. longa but are less abundant than in E. gracilis.
Inefficient Processing: RBCS monomers are undetectable in E. longa, suggesting inefficient processing of the precursor polyprotein.
Post-Transcriptional Regulation: RBCS abundance is regulated post-transcriptionally, with rapid degradation upon blocking cytoplasmic translation.

Further analysis indicates that while both the large (RBCL) and small (RBCS) subunits of RuBisCO are produced in E. longa, their abundance is notably low compared to E. gracilis. Even more striking is the absence of detectable RBCS monomers in E. longa, suggesting that the precursor polyprotein is not efficiently processed into its functional components. This inefficiency is compounded by post-transcriptional regulation, where the abundance of RBCS is carefully controlled. Blocking cytoplasmic translation in E. longa leads to rapid degradation of the protein, unlike the stable RBCS in photosynthetic E. gracilis.

Evolutionary Degradation or Unorthodox Role?

Collectively, these results point towards the evolutionary degradation of RuBisCO in E. longa, suggesting that the enzyme is becoming defunct. However, the persistence of the rbcL gene and the continued synthesis of both RuBisCO subunits imply that it may still play a biological role in this species, perhaps one that is rather unorthodox. Future research will be necessary to fully elucidate the function, if any, of RuBisCO in E. longa and other non-photosynthetic organisms.