This shared evolutionary adaptation may have been crucial for the establishment of environmental conditions required for the emergence of humankind.

For decades, scientists have debated whether there is a common evolutionary origin for the different photosynthetic organisms present today.

Reporting in today’s Nature, scientists from the Wolfson Laboratories, Department of Biological Sciences, Imperial College, now provide evidence for a link.

They have discovered a new protein supercomplex in the photosynthetic pathway that links two major proteins that were previously thought to work autonomously.

The key proteins Photosystem I (PSI) and Photosystem II (PSII), work together in the photosynthetic pathway to produce oxygen and energy for plants to grow.

The Imperial researchers investigated the possibility of this link using cyanobacteria, a major photosynthetic producer in the world’s oceans.

Tom Bibby and colleagues were investigating the role of a PSII-like protein that is produced by cyanobacteria in conditions of low-iron availability. They expected this protein to interact with PSII, due to its DNA sequence similarity with one of its proteins.

By recreating "iron-stress response" conditions in cyanobacteria, the team found that this PSII-like protein interacts, surprisingly, with PSI, by forming a light harvesting antenna of 18 chlorophyll molecules around the protein complex.

The presence of the antenna increases the light harvesting ability by approximately 72 per cent compared with that of the normal PSI alone.

This means that cyanobacteria can produce oxygen even in low iron conditions. This adaptation would have global environmental significance -- both for creating the levels of oxygen in the atmosphere that allowed the evolution of humans and maintaining them to this day.

Professor Jim Barber, senior author of the paper and head of the Photosynthesis Research Group, says, “This is a staggering finding. It is the first time that an antenna ring of chlorophyll molecules has been found in oxygen producing organisms.

"You don’t have discoveries like this everyday. It means a whole new discussion of how light in aquatic environments is absorbed over massive areas, such as the oceans -- both at the surface and deep within the ocean.

“The increase in antenna size is almost certainly a response to the reduction in the level of ‘light harvesting proteins’ such as PSI complexes which need iron for their synthesis and assembly. This stress response allows cyanobacteria to produce oxygen even in conditions of low iron availability.”

Cyanobacteria have been a major photosynthetic producer in the world’s oceans for three billion years. They used the abundant iron in the primeval oceans to synthesise and assemble the PSI and PSII protein complexes.

With time, iron became extremely scarce in the oceans and the cyanobacteria, which are limited in their photosynthetic activity by the availability of iron, had to compensate for this loss in some way.

The PSI supercomplex was visualized by high-resolution electron microscopy. Dr Jon Nield, an author on the paper, modeled known X-ray diffraction derived structures into the calculated PSI supercomplex determined by these electron microscopy-based techniques. This allowed a better understanding of how the light harvesting antenna ring of PSII-like protein interacted with PSI.

Professor Jim Barber says, “The discovery of this PSI supercomplex and its association with a PSII-like protein is surprising, but it finally suggests that an evolutionary link between the two photosynthetic complexes does exist.”

The Photosynthesis Research Group, headed by professor Jim Barber, Department of Biological Sciences, is focusing its efforts on a key component of the photosynthetic process known as Photosystem II . This Photosystem, consisting of several proteins, uses light energy absorbed by chlorophyll to split water.

The main goal is to understand the molecular reactions involved in water splitting and to use such knowledge not only to devise possible energy sources for the future, but as a basis for genetically engineering new crop cultivars able to survive and grow more robustly in environments which hitherto were hostile.

(Reference: Nature 16 August 2001 Volume 413. Iron-stress induces the formation of an antenna ring around trimeric Photosystem I in cyanobacteria. Thomas S. Bibby, Jon Nield & James Barber, Wolfson Laboratories, Department of Biological Sciences, Imperial College of Science, Technology and Medicine, London.)

Related website:

Imperial College of Science, Technology and Medicine

16-Aug-2001