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In symbiosis with alfalfa
The complex genome sequence of Sinorhizobium meliloti
By Birgit Hofmann-Reinert

S. meliloti cells

An international team of researchers has sequenced the genome of Sinorhizobium meliloti, a common soil bacterium that lives on the roots of alfalfa plants and other legumes. Analyses of the genome sequence revealed some of the mechanisms involved in the organism's symbiotic relationship with alfalfa and its ability to transform atmospheric nitrogen into a form that plants can use for growth. Bacteria such as S. meliloti not only provide nitrogen to their plant host, but also leave behind excess nitrogen in the soil, potentially reducing the need for fertilizers.

Sinorhizobium meliloti possesses a complex genome consisting of three circular elements: It is composed of a single chromosome and two smaller strands of DNA, called pSymA and pSymB respectively. A complete map of the genome sequence of strain 1021, which contains about 6,7 million base pairs, is published in Science.

Alfalfa plant (Medicago sativa)

Analyses of the three strands of DNA—or replicons—of S. meliloti suggest that each has distinctive structural and functional features. But all replicons contribute, in varying degrees, to symbiosis with the plant host, the researchers found. The tripartite architecture led the researchers to form three teams, each of which sequenced one of the replicons. The results are published in three papers in Proceedings of the National Academy of Sciences.

Analysis of the chromosome sequence revealed that this replicon carries all housekeeping genes (such as genes involved in metabolic pathways) as well as genetic information important for plant interaction, stress responses, mobility, and chemotaxis processes. The chromosome study was led by Francis Galibert of the Faculté de Médecine in Rennes, France.

The second replicon, pSymB, is reponsible for nutrient uptake and successful invasion of the plant host. pSymB was sequenced by Turlough M. Finan, of McMaster University in Hamilton, Canada, and colleagues.

Mature, nitrogen-fixing nodules on an alfalfa root.

The smallest replicon, pSymA, was mapped by Sharon R. Long, of Stanford University, California, and colleagues. pSymA carries the nitrogen-fixing genes the bacterium needs for converting atmospheric nitrogen gas. "pSymA is clearly specialized for nodulation and nitrogen fixation," the researchers write.

Thin section of an alfalfa nodule. An infection thread is visible as a white-blue fluorescing structure in the outer layers (at the top).

Sinorhizobium meliloti and alfalfa can grow in isolation from each other. But when brought together, they enter into a symbiotic relationship. The bacterium enters the root tissue via infection threads and forms growths (nodules) on the roots of the alfalfa plants. Inside the nodules, it converts atmospheric nitrogen into ammonia, a nitrogen-based compound plants can use to produce proteins. This process is called nitrogen fixation. In return, the plants provide a carbon and energy source for the bacteria.

The association between S. meliloti and alfalfa is considered one of the leading model systems for nitrogen fixation and symbiosis studies, according to the researchers. The new genome map could provide the basis for improving crop yields and enhancing soil fertility without the expensive and problematic use of fertilizers.

The Sinorhizobium meliloti strain 1021 Genome Project can be viewed at

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Barnett, M.J. et al. Nucleotide sequence and predicted functions of the entire Sinorhizobium meliloti pSymA megaplasmid. Proc Natl Acad Sci USA, Early Edition (July 31, 2001).
Capela, D. et al. Analysis of the chromosome sequence of the legume symbiont Sinorhizobium meliloti strain 1021. Proc Natl Acad Sci USA, Early Edition (July 31, 2001).
Finan, T.M. et al. The complete sequence of the 1,683-kilobase pSymB megaplasmid from the N2-fixing endosymbiont Sinorhizobium meliloti. Proc Natl Acad Sci USA, Early Edition (July 31, 2001).
Galibert, F. et al. The composite genome of the legume symbiont Sinorhizobium meliloti. Science 293, 668-672 (July 27, 2001).

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