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Ensifer medicae WSM419
   
   
 

Natural and agricultural systems are nitrogen deficient and therefore often depend for productivity upon atmospheric nitrogen fixed in nodules formed on legumes by root nodule bacteria. The commonly accepted figure for global nitrogen fixation in agriculture is 25-90 million metric tons, worth in excess of U.S. $10 billion. Rhizobia associated with annual pasture legumes contribute a substantial proportion of this fixed nitrogen, and the amount of nitrogen fixed annually by the Sinorhizobium-Medicago symbioses is estimated to be worth $250 million. Two sinorhizobial species are the focus of intensive research: S. meliloti and S. medicae. The latter strain can be distinguished from the former by its unique capacity to fix nitrogen in association with annual and perennial Medicago hosts (including alfalfa) of worldwide agronomic value. DNA hybridization experiments reveal no more than 60% homology between the two species, explaining many physiological and genetic differences. Thus, the available S. meliloti Sm1021 sequence, by itself, is of limited value for studies concerning S. medicae.

JGI will sequence the well-characterized WSM419 strain of S. medicae, which is superior in nitrogen fixation and acid tolerance. The sequence information, together with that for S. meliloti, will reveal information regarding genome architecture that is vital for global insights into cell function. This information could provide a detailed understanding of the physiology and genetics of the two species. The S. medicae genome sequence obtained will also be compared to the sequences available for other related bacteria, providing a tremendous resource for understanding the root-nodule bacteria.

Once the S. medicae sequence is available, it will be possible to develop microarray chips to provide a global picture of gene expression in this species (its "transcriptome"). Comparing transcription profiles of S. medicae, S. meliloti, and A. tumefaciens will provide scientists with the capacity to identify similarities and crucial differences in gene expression in response to specific stimuli. Such transcriptome comparisons should reveal an exciting insight into genes involved with nodulation specificity, molecular signaling, nitrogen fixation, pH response, and virulence, potentially producing outcomes of benefit to worldwide agriculture.