Survival tactics in bacteria - environmental conditions fit for mankind

August 15, 2001

Scientists from Imperial College, London, have made an important evolutionary link between the two powerhouse protein complexes that drive photosynthesis. 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 (1) organisms present today.

Reporting in today's Nature journal, 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 (2).

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 (3). 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 primaeval oceans to synthesise and assemble the PSI and PSII protein complexes (4). 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 visualised by high-resolution electron microscopy (5). Dr Jon Nield, an author on the paper, modelled 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: "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."
-end-
Notes to editors:

Nature 16 August 2001 Volume 413

Title of paper: Iron-stress induces the formation of an antenna ring around trimeric Photosystem I in cyanobacteria.

Authors: Thomas S. Bibby, Jon Nield & James Barber Wolfson Laboratories, Department of Biological Sciences, Imperial College of Science, Technology and Medicine, London SW7 2AY, UK.

1. Photosynthesis is the process by which plants, algae and certain bacteria convert atmospheric carbon dioxide to organic materials. They do so by using light energy from the sun to split water into oxygen and hydrogen. The oxygen is lost to the atmosphere as a by-product while the hydrogen equivalents are used to 'fix' carbon dioxide.

The complexes PSI and PSII (see below) perform photosynthesis - where light energy is used to split water, which ultimately produces the oxygen we breathe, and to create organic compounds.

2. Photosystem II (PSII) - is a protein complex found in some photosynthetic organisms such as cyanobacteria and higher plants. In the photosynthesis process, it uses light energy to split water into the atmospheric oxygen we breathe. PSII is also involved in the production of a substantial proportion of the global biomass - that is, it provides the reducing equivalents required to produce food for biological organisms.

Photosystem I (PSI) - is also involved in the photosynthesis process where its reactions lead to the creation of organic matter.

3. Chlorophyll is the green pigment in plants that can capture light prior to photosynthetic reactions taking place.

4. Iron is the fourth most abundant element in the Earth's crust. Yet, its level in the aquatic ecosystems, particularly the open oceans where most cyanobacteria are found, is low. Iron is required by photosynthetic complexes to carry out their photosynthetic functions. A lack of utilisable iron limits photosynthetic activity and phytoplankton growth.

5. Electron microscopes use beams of electrons rather than beams of light. They are much more powerful than conventional microscopes, allowing components of cells, including the organelles, to be visualised, even down to individual atoms.

6. The Photosynthesis Research Group, headed by professor Jim Barber, Department of Biological Sciences, Imperial College, London, 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. Website: www.bc.ic.ac.uk

7. Imperial College of Science, Technology and Medicine is the largest applied science, technology and medicine university institution in the UK. It is consistently rated in the top three UK university institutions for research quality, with one of the largest annual turnovers (UKP339 million in 1999-2000) and research incomes (UKP176 million in 1999-2000). Website: www.ic.ac.uk

Imperial College London

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