What is RuBisCO, and why is it so important?

RuBisCO, short for ribulose-1,5-bisphosphate carboxylase/oxygenase, is an enzyme critical for photosynthesis. It captures carbon dioxide to produce sugars, essentially kicking off the process that sustains most life on Earth. Its significance lies in its role as the primary enzyme responsible for carbon fixation, making it a cornerstone of the global carbon cycle. Without RuBisCO, plants and algae couldn't convert atmospheric carbon dioxide into energy-rich molecules, with huge implications for food chains and the atmosphere.

What is Euglena longa, and what makes it interesting to scientists?

Euglena longa is a close relative of the photosynthetic alga Euglena gracilis. However, unlike Euglena gracilis, Euglena longa has a non-photosynthetic plastid, meaning it cannot perform photosynthesis. This makes it an interesting subject for study because scientists are trying to understand why it retains certain photosynthetic machinery, like the RuBisCO enzyme, even though it can no longer photosynthesize.

How is the RuBisCO enzyme different in Euglena longa compared to photosynthetic organisms?

In Euglena longa, the RuBisCO enzyme differs significantly from its photosynthetic relatives. The genetic sequence encoding the large subunit of RuBisCO (RBCL) is highly divergent, suggesting that its structure and function may have changed. Additionally, while it produces the small subunit of RuBisCO (RBCS) as a polyprotein like its relatives, one of the repeats is highly divergent, affecting how the RuBisCO complex assembles. Also, in Euglena Longa the protein is less abundant and less stable.

What do scientists think might be the future role of RuBisCO in Euglena longa?

Scientists believe that RuBisCO in Euglena longa may be evolving to perform a different function unrelated to carbon fixation. It could be involved in other metabolic processes or have a structural role within the plastid. The enzyme's adaptability highlights the remarkable ability of organisms to repurpose existing tools to thrive in new environments. Research into these changes can provide insights into how life adapts and diversifies.

What is a polyprotein, and how does it relate to RuBisCO in Euglena longa?

A polyprotein is a long chain of multiple repeats of the RBCS. In Euglena longa, the small subunit of RuBisCO (RBCS) is produced as a polyprotein. However, one of the repeats within this polyprotein is highly divergent, which impacts how the RuBisCO complex assembles and functions. The divergence and inefficient processing of this polyprotein is thought to compromise the formation of a functional RuBisCO enzyme.

Illustration of a cell with a non-photosynthetic plastid and a distorted RuBisCO enzyme, symbolizing enzyme evolution.

The Curious Case of RuBisCO in a Non-Photosynthetic Alga: What It Means for Evolution

Lena Kashyap in Science & Nature September 2025 • 4 min read.

"Scientists uncover surprising adaptations in Euglena longa, challenging our understanding of cellular biology and hinting at the flexible roles of essential enzymes."

The plastid, a tiny compartment within plant and algal cells, is famous for photosynthesis. This process, essential for life on Earth, uses the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, commonly known as RuBisCO, to capture carbon dioxide and kickstart the production of sugars. Think of RuBisCO as the unsung hero behind every breath of fresh air and bite of food.

But what happens when an organism loses its ability to photosynthesize? That's the puzzle scientists are exploring with Euglena longa, a close cousin of the photosynthetic alga Euglena gracilis. Unlike its green relative, Euglena longa has a non-photosynthetic plastid, raising questions about the purpose of retaining RuBisCO when it can no longer perform its primary function.

A recent study sheds light on this mystery, revealing that RuBisCO in Euglena longa has undergone significant changes at the molecular level. These changes suggest a potential shift in function, opening new avenues for understanding how organisms adapt and repurpose essential enzymes in response to environmental pressures.

The Divergent Evolution of RuBisCO: What Makes It Different in Euglena longa?

Illustration of a cell with a non-photosynthetic plastid and a distorted RuBisCO enzyme, symbolizing enzyme evolution.

The study found that the RuBisCO found in Euglena longa differs significantly from the RuBisCO in its photosynthetic relatives. The genetic sequence encoding the large subunit of the RuBisCO enzyme (RBCL) is highly divergent, suggesting that the protein's structure and function may have changed over time. This divergence is particularly interesting because RuBisCO is typically highly conserved across species, owing to its critical role in photosynthesis.

To add another layer to the puzzle, scientists discovered that Euglena longa, like Euglena gracilis, produces the small subunit of RuBisCO (RBCS) as a polyprotein—a long chain of multiple RBCS repeats. One of these repeats, however, is highly divergent, indicating an unusual modification in the assembly or function of the RuBisCO complex.

Here are the key differences in RuBisCO between photosynthetic and non-photosynthetic euglena:

RBCL Sequence: Highly divergent in Euglena longa, suggesting functional shift.
RBCS Polyprotein: Includes a divergent repeat, impacting complex assembly.
Protein Abundance: Significantly lower in Euglena longa compared to Euglena gracilis.
RBCS Processing: Inefficient in Euglena longa, hindering monomer production.
Protein Stability: RBCS unstable in Euglena longa when translation is blocked.

These findings indicate that RuBisCO in Euglena longa is not only structurally different but also less abundant and less stable than its photosynthetic counterpart. The inability to detect RBCS monomers suggests that the polyprotein is not efficiently processed, further compromising the formation of a functional RuBisCO enzyme.

The Future of RuBisCO: A Glimpse into Evolutionary Adaptation?

These findings suggest that RuBisCO may be evolving towards a different role in Euglena longa, one that is perhaps unrelated to carbon fixation. It is possible that RuBisCO is involved in other metabolic processes or that it plays a structural role within the plastid. Further research will be needed to fully understand the function, if any, of RuBisCO in this non-photosynthetic alga. The story of RuBisCO in Euglena longa highlights the remarkable adaptability of life and the ability of organisms to repurpose existing tools to thrive in new environments. By studying these evolutionary adaptations, we can gain deeper insights into the fundamental processes that drive the diversity of life on Earth.