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Nitrogen Metabolism in Sinorhizobium meliloti–Alfalfa Symbiosis: Dissecting the Role of GlnD and PII Proteins

March 2012 , Volume 25 , Number  3
Pages  355 - 362

Svetlana N. Yurgel,1 Jennifer Rice,1 and Michael L. Kahn1,2

1Institute of Biological Chemistry and 2School of Molecular Biosciences, Washington State University, Pullman 99164-6340, U.S.A.

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Accepted 1 November 2011.

To contribute nitrogen for plant growth and establish an effective symbiosis with alfalfa, Sinorhizobium meliloti Rm1021 needs normal operation of the GlnD protein, a bifunctional uridylyltransferase/uridylyl-cleavage enzyme that measures cellular nitrogen status and initiates a nitrogen stress response (NSR). However, the only two known targets of GlnD modification in Rm1021, the PII proteins GlnB and GlnK, are not necessary for effectiveness. We introduced a Tyr→Phe variant of GlnB, which cannot be uridylylated, into a glnBglnK background to approximate the expected state in a glnD-sm2 mutant, and this strain was effective. These results suggested that unmodified PII does not inhibit effectiveness. We also generated a glnBglnK-glnD triple mutant and used this and other mutants to dissect the role of these proteins in regulating the free-living NSR and nitrogen metabolism in symbiosis. The glnD-sm2 mutation was dominant to the glnBglnK mutations in symbiosis but recessive in some free-living phenotypes. The data show that the GlnD protein has a role in free-living growth and in symbiotic nitrogen exchange that does not depend on the PII proteins, suggesting that S. meliloti GlnD can communicate with the cell by alternate mechanisms.

© 2012 The American Phytopathological Society