What is RuBisCO and what role does it typically play in cells?

RuBisCO, short for Ribulose-1,5-bisphosphate carboxylase/oxygenase, is an enzyme found in plant and algal cells. Its primary role is to catalyze the initial step of the Calvin-Benson cycle, where it converts carbon dioxide into organic compounds, essentially initiating photosynthesis. However, RuBisCO also catalyzes a competing reaction leading to photorespiration, making it a key player in carbon fixation but also potentially a source of inefficiency.

Why is the presence of RuBisCO in Euglena longa considered unusual?

The presence of RuBisCO in Euglena longa is considered unusual because this alga has a non-photosynthetic plastid. RuBisCO is typically essential for photosynthesis. Since E. longa doesn't perform photosynthesis, the retention of the rbcL gene (which encodes the large subunit of RuBisCO) and the presence of RuBisCO raise questions about its function and whether it has been repurposed for a different biological role within the organism.

How does the structure of RuBisCO in Euglena longa differ from that in photosynthetic algae?

In Euglena longa, the RBCL protein, the large subunit of RuBisCO, exhibits substantial differences in its protein sequence compared to photosynthetic relatives. Additionally, the small subunit of RuBisCO, RBCS, is encoded as a precursor polyprotein with multiple repeats. The abundance of both RBCL and RBCS proteins is significantly lower in E. longa compared to photosynthetic species like E. gracilis. Moreover, the precursor polyprotein isn't processed effectively, and the abundance of RBCS is regulated post-transcriptionally, with the protein quickly degrading if translation is blocked. These differences suggest RuBisCO is undergoing evolutionary degradation in E. longa.

What are the implications of the observed differences in RuBisCO in Euglena longa?

The observed differences in RuBisCO in Euglena longa suggest that the enzyme may be losing its photosynthetic function and potentially adopting a different role. The low protein abundance, inefficient processing, and post-transcriptional regulation of RBCS all point towards a reduction in its activity and functionality related to photosynthesis. These findings suggest that E. longa might be in a transitional phase, gradually losing its photosynthetic capabilities and adapting RuBisCO for other purposes, or it could become non-functional.

What further research is needed to fully understand the role of RuBisCO in Euglena longa?

To fully understand the role of RuBisCO in Euglena longa, further research is crucial. This includes investigating the potential non-photosynthetic functions of RuBisCO, exploring the specific roles of the RBCL and RBCS subunits within the alga, and studying the mechanisms of post-transcriptional regulation. Additionally, comparing RuBisCO in various related species can provide insights into its evolutionary trajectory and the adaptability of cellular machinery. This research will help uncover how E. longa adapts and innovates at the cellular level.

Surreal illustration of Euglena longa with altered RuBisCO enzyme.

Euglena Longa's RuBisCO Mystery: What Does a Non-Photosynthetic Alga Do With a Photosynthesis Enzyme?

Nico Varela in Science & Nature September 2025 • 4 min read.

"Scientists explore the divergent features and regulation of RuBisCO in Euglena longa, revealing surprising insights into its potential non-photosynthetic roles."

The plastid, a vital component within plant and algal cells, operates as a semi-autonomous organelle. Its function is deeply intertwined with the coordinated expression of genes located both in the nucleus and within the plastid itself. A prime example of this coordination is Ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known as RuBisCO. This enzyme catalyzes the initial step of the Calvin-Benson cycle, where carbon dioxide is converted into organic compounds, effectively kick-starting the process of photosynthesis.

RuBisCO's role extends beyond carbon fixation; it also catalyzes a competing reaction that leads to photorespiration. The enzyme, in its complete form, consists of eight subunits of two distinct types. While exceptions exist in certain algae, where RuBisCO is composed of a single subunit, the large subunit, crucial for the enzyme's catalytic activity, is generally encoded in the plastid genome by the rbcL gene. This subunit is synthesized within the plastid's stroma.

The small subunit, encoded by the rbcS gene, is located in the plastid genome in specific groups like glaucophytes and rhodophytes. However, in green algae and plants, this subunit is encoded in the nucleus, synthesized in the cytoplasm, and then imported into the plastid. While its precise functions are still under investigation, the small subunit is believed to play a role in stabilizing the holoenzyme and ensuring its maximal catalytic activity.

The Curious Case of RuBisCO in Non-Photosynthetic Algae

Surreal illustration of Euglena longa with altered RuBisCO enzyme.

Euglena longa, a close relative of the photosynthetic model alga Euglena gracilis, presents a unique puzzle. While E. longa possesses a non-photosynthetic plastid, its genome retains the rbcL gene, responsible for encoding the large subunit of RuBisCO. This retention raises questions about the enzyme's function in an organism that doesn't perform photosynthesis.

To investigate this, researchers delved into the characteristics of RuBisCO in E. longa, comparing it to its photosynthetic counterparts. Their findings revealed significant differences, particularly in the protein sequence of the large subunit (RBCL), suggesting a potential shift in function due to the loss of photosynthetic activity.

Divergent RBCL Sequence: The RBCL protein in E. longa exhibits substantial differences compared to photosynthetic relatives.
RBCS Polyprotein Structure: Similar to E. gracilis, E. longa encodes the small subunit of RuBisCO (RBCS) as a precursor polyprotein with multiple repeats, one of which is highly divergent.
Low Protein Abundance: Both RBCL and RBCS proteins are produced in E. longa, but their levels are significantly lower than in E. gracilis.
Inefficient Processing: No RBCS monomers were detected in E. longa, suggesting that the precursor polyprotein isn't being processed effectively.
Post-Transcriptional Regulation: The abundance of RBCS is regulated after transcription. Blocking translation quickly degrades the protein in E. longa, unlike in photosynthetically grown E. gracilis.

These results indicate that RuBisCO in E. longa is undergoing evolutionary degradation, potentially becoming non-functional. Its biological role in this species may be unconventional, if it exists at all. The study suggests that E. longa might be in a transitional phase, gradually losing its photosynthetic capabilities and adapting RuBisCO for other purposes.

Unraveling the Mystery

Further research is necessary to fully understand the role of RuBisCO in E. longa. By exploring the potential non-photosynthetic functions of this enzyme, we can gain new insights into the adaptability and evolutionary trajectory of cellular machinery. The study of E. longa provides a window into the complex processes that drive cellular adaptation and innovation.