Green plants have the amazing ability to make their own food.

Unlike humans and animals, who must eat plants and other animals, plants gather energy from sunlight and carbon dioxide from air, and convert them into carbon compounds - food - through photosynthesis. In the process, they create the food and oxygen that sustain us.

"Plants do all of this for us. It seems the least we can do for them is to study how they do it," said Robert Spreitzer, an Institute of Agriculture and Natural Resources molecular geneticist in the biochemistry department.

Spreitzer has spent 18 years doing just that: studying an enzyme that is Earth's most abundant protein and a key to the mysteries of photosynthesis.

The enzyme, ribulose-1,5-biphosphate carboxylase/oxygenase, is commonly called Rubisco. Rubisco acts as a catalyst in photosynthesis, causing carbon dioxide (CO2) to combine with a five-carbon sugar.

In simpler terms, Rubisco is necessary for fixing CO2 - taking CO2 from the air and turning it into the carbon compounds that make up the plants that become our food and clothing.

Spreitzer is exploring ways to design a Rubisco that could fix more CO2. In crop plants this could translate into higher yields without the addition of expensive inputs, such as fertilizer.

"We're on an expedition to figure out what parts of Rubisco are important for attributes like the efficiency of CO2 fixation," Spreitzer said. "In the process, we may learn enough to design a better enzyme."

His quest began as classical genetics work on Rubisco in the one-celled green alga, Chlamydomonas. Chlamydomonas offers a unique trait. Certain types contain genetic mutations that block their ability to photosynthesize. Rather than dying from lack of food, these mutants can be kept alive in laboratories on a carbon-based growth medium.

Using what he calls a "brute force" approach of genetically screening tens of thousands of Chlamydomonas colonies, Spreitzer found 10 Rubisco mutants.

"First you make mutants, then you screen these mutants looking for a second mutation that fixes the first one," Spreitzer said. "These second-site mutations let you identify important structural and functional regions of the enzyme."

These mutants are vital tools enabling Spreitzer and other scientists to learn about Rubisco's genetics and function in Chlamydomonas. The mutants are invaluable to understanding how Rubisco's structure influences its CO2-fixing efficiency.

One limit to Rubisco's efficiency is that it often mistakenly captures the much more abundant oxygen (O2) molecule, instead of CO2. While crop plants are better than Chlamydomonas at discriminating between CO2 and O2, they're slower at fixing CO2.

Spreitzer and his University of Nebraska students want to design a better enzyme by swapping parts of highly efficient enzymes for the slower, poorly discriminating enzyme parts.

Spreitzer knew from his genetics research and from looking at the structure of the enzyme which part of Rubisco is most important in discriminating between CO2 and O2.

"This allowed us to narrow our search," he said. But when they genetically transformed Chlamydomonas Rubisco to have the same structure as the crop enzyme, the combination didn't create a more efficient enzyme - it actually was less efficient.

"Maybe we need to make the crop plant Rubisco more like Chlamydomonas. That's what we're trying to figure out - what controls these things," Spreitzer said.

Spreitzer's team continues exploring ways to enhance Rubisco, using a combination of genetics, biochemistry and molecular biology.

"We haven't made Rubisco better yet, but we know more about it," Spreitzer said. Gaining this kind of knowledge is what basic science is about.

"Basic science is an investment in the future. Without it, we will never have the knowledge to make practical applications."

Competitive grants from the U.S. Department of Energy and the U.S. Department of Agriculture National Research Initiative help fund this research.

- Monica Manton Norby

IANR Molecular Geneticist Robert Spreitzer uses a molecular graphics workstation to produce a three-dimensional atomic image of the Rubisco enzyme. He's wearing special glasses to view the image in 3-D. He studies this enzyme's role in photosynthesis.

In this computer graphic, each ball represents an atom. The pink and purple balls are enzyme parts Spreitzer identified through selection and screening as potential targets for engineering a more efficient Rubisco.

This ribbon diagram is a simplified view of a crop plant Rubisco, showing only the amino acids that make up the enzyme's "backbone." Red areas mark where the crop plant enzyme differs from Rubisco in the one-celled alga, Chlamydomonas. Genetics research helps scientists decide which area to target in enzyme design.