Brussels, 27 July 2001
Key words: genome, plants, agriculture, fertilisers
EMBARGO: 26 July 2001 at 19.00 (CET)
European, Canadian and American scientists will publish tomorrow in Science the complete genetic sequence of a nitrogen-fixing bacteria, Sinorhizobium meliloti. This achievement heralds a better understanding of the mechanisms which allow these bacteria to fix atmospheric nitrogen and transfer it to plants. In the long term it might help scientists transfer this capacity to plants, allowing them to fix nitrogen for themselves, thus avoiding the need for nitrogen fertilisers. This would be a step towards 'green agriculture'.
The EU's Research Commissioner, Philippe Busquin, points out that, 'This project demonstrates the contribution of modern biotechnology to sorting out some of our current environmental problems. By nature, these problems need to be tackled at a European level. EU-funded research provides the tools and general framework to help European science innovate more easily and more rapidly.'
Project co-ordinator Francis Galibert, from the University of Rennes (France) believes: 'This work is important because of the nature of the bacterium which has been analysed and its role in our living world. Initiated in Europe, the project shows also how useful European funding is - in this case enabling several laboratories to form a network. I hope that Europe will maintain its support for sequencing small genomes of interest for biotechnology.'
Nitrogen is essential for plants to grow, and this is supplied to them in the form of ammonium ions NH4+, provided by nitrogen fertilisers or by reduction of atmospheric nitrogen through various natural processes. As a result, nitrogen fertilisers represent a huge economic market; in 2000 for example, more than 80 million tons were manufactured by the chemical industry, representing a total value of more than EUR 15 billion.
However, nitrogen from fertilisers represents only 30% of the total. The remaining 70% comes from natural phenomenon - almost half of this (or 30% of the overall total) from symbiosis between legumes and bacteria collectively named Rhizobium. Through this symbiosis, the bacterium transforms atmospheric nitrogen into ammonium ions, some of which are used by plants to synthesise proteins and nucleic acids.
The rest of the ammonium ions synthesised by the bacteria are released into the soil, contributing to its enrichment, which explains and justifies traditional crop rotation. A better understanding of 'legume-rhizobium' symbiosis is therefore essential to achieve the development of sustainable and environmentally-friendly agriculture.
A model system
Sinorhizobium meliloti is a symbiotic micro-organism associated with many host-legumes which are important in agronomy, such as lucerne (Medicago sativa). The association S. meliloti / Medicago truncatula was selected many years ago by scientists as a model for the study of the symbiosis of plants with micro-organisms, and of nitrogen fixing. S. meliloti is also biologically close to some pathogens of plants (Agrobacterium) and animals (Brucella).
It was for these reasons that a consortium of European laboratories, supported by EUR 2 442 000 of European Union funding, and two laboratories in North America (in California and Canada) came together to fully sequence the Sinorhizobium meliloti genome (see attachment).
As for many Rhizobium, S. meliloti has a relatively large genome which is divided into three parts: a chromosome which is 3.7 million base pairs long (3 654 137 bp) and two megaplasmids, pSyma et pSymb, whose lengths are 1.7 million and 1.4 million base pairs respectively (1 638 332 bp and 1 354 226 bp). Quite surprisingly, this genome contains 6204 genes coding for proteins (thus more than in the yeast S. cerevisiae), representing nearly 87% of the full sequence with very few repeated sequences (about 2.2%). This may indicate that S. meliloti is genetically equipped to adapt itself to a wide range of metabolic and regulation conditions.
For 60% of the genes, sequence comparisons with genes already identified in other micro-organisms, or more complex organisms, have allowed researchers to suggest biochemical and biological functions, which obviously need to be tested experimentally. The distribution of these genes in the genome reveals a certain degree of specialisation. Indeed, most of the key functions of the cell are coded by genes located on the chromosome while the pSymb plasmid is particularly rich in genes coding for proteins involved in transport or synthesis of polysaccharides (important for making the bacterium membranes). As far as the second plasmid is concerned, pSyma contains most of the genes involved in nitrogen fixing and nodulation. However, certain functions such as those corresponding to adaptation or to stress-induced response seem to be well spread across the genome.
As far as the remaining 40% of genes are concerned, scientists are unable to propose any function at the moment. Additional work is therefore necessary to confirm through biological experiments the functions proposed for 60 % of the genes and, on the other hand, to identify the functions of the others. It is indeed possible that a more comprehensive analysis of the metabolic functions and of the regulation and interconnection patterns of all these genes will lead to a better understanding of nitrogen fixing, a key process for a large part of the living world.
For further information related to the projects presented, please contact:
Ati Vassaritti, Quality if Life Unit,
Tel: +32.2.295.83.09; Fax:+32.2.299.18.60
· Francis Galibert, Université de Rennes (F),
Tel : +33.2.99 33 62 16 or 33.6.08 24 72 43, Fax : +33.2.99 33 62 00
E-mail : firstname.lastname@example.org
Michel Claessens, Communication and Information Officer,
Tel : +32.2.295 99 71; Fax: +32.2.295 82 20
Partners of the Melito project
External costs (for electricity production in the EU (in cent/kWh**, PV = photovoltaics)
The project has been supported by the European Union (Biotechnology programme) and involved six European teams:
- Laboratoire de Génétique et Développement UMR6061-CNRS, Faculté de Médecine, 2 avenue du Pr. Léon Bernard, F-35043 Rennes cedex: Francis Galibert (co-ordinator)
- Laboratoire de Biologie Moléculaire des Relations Plantes-Microorganismes, UMR215-CNRS-INRA, Chemin de Borde Rouge, BP27, F-31326 Castanet Tolosan Cedex, France: Jacques Batut
- Unité de Biochimie Physiologique, Université Catholique de Louvain, Place de la Croix du Sud 2, BTE 20, B- 1348 Louvain la Neuve, Belgique: André Goffeau
- GATC GmbH, Fritz-Arnold-str. 23, D-78467 Konstanz, Germany: Thomas Pohl
- Unité de Microbiologie, Faculté des Sciences Agronomiques de Gembloux, Avenue Maréchal Juin 6, B-5030 Gembloux, Belgium: Daniel Portetelle
- Universität Bielefeld, Biologie IV (Genetik), Universitätstr. 25, D-33615 Bielefeld, Germany: Alfred Pühler
As well as two laboratories in North America:
- Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario, L8S 4K1 Canada: Turlough M. Finan
- Department of Biological Sciences, Stanford University, Stanford, CA 94305 and Howard Hughes Medical Institute, Stanford University, Stanford, CA 94305, USA: Sharon R. Long