Brussels, 02/08/2001 (Agence Europe) - A research project, supported by the European Commission to the amount of 2.442 million euros, has enabled European, Canadian and American scientists to carry out full genetic sequencing of a nitrogen fixative bacteria, Sinorhizobium meliloti (S.meliloti). This result, which has just been published in the review "Science" is important for the understanding of the fixation of nitrogen by plants but which may also be useful elsewhere to the extent that S.meliloti is biologically and phylogenetically close to certain pathogenic bacteria of plants, such as Agrobacterium, or of animals, such as Brucella.
Just as numerous Rhizobium do, S. meliloti possesses a complex genome in three parts: a chromosome of 3.7 million pairs of bases and two megaplasmides, respectively of 1.7 and 1.4 million pairs of bases. Unexpectedly this genome contains 6,204 coding genes for proteins and an extremely low number of repeated sequences (2.2%) which suggests this bacteria is genetically equipped to satisfy a rich metabolic and regulatory repertoire. This sequencing has enabled the identification of genes (60% in total) for which a supposed function (transport or synthesis of polysaccharides; fixation of nitrogen…) is still yet to be proven. For the 40% of remaining genes, no hypothesis of function can be currently put forward and new investigations are needed. Present in numerous leguminous plants, notably lucerne, S. meliloti belongs to a family of bacteria which transform atmospheric nitrogen into ammonium ions, a part of which is used by the plant for the synthesis of proteins and nucleic acids. The other part is freed into the ground and contributes to its enrichment. A better knowledge of the mechanisms of this bacteria could then contribute over time to a renewable and more environmentally-respecting agriculture through a reduction in the use of nitrogenous fertilisers whose consumption has reached record levels (for the year 2000, more than 80 million tonnes have been produced by the chemical industry at a cost of over 15 billion euros).