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Applied and Environmental Microbiology, April 2006, p. 2738-2748, Vol. 72, No. 4
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.4.2738-2748.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Identification of Sinorhizobium meliloti Early Symbiotic Genes by Use of a Positive Functional Screen

Xue-Song Zhang and Hai-Ping Cheng^*

Biological Sciences Department, Lehman College, The City University of New York, Bronx, New York 10468

Received 15 August 2005/ Accepted 4 February 2006

The soil bacterium Sinorhizobium meliloti establishes nitrogen-fixing symbiosis with its leguminous host plant, alfalfa, following a series of continuous signal exchanges. The complexity of the changes of alfalfa root structures during symbiosis and the amount of S. meliloti genes with unknown functions raised the possibility that more S. meliloti genes may be required for early stages of the symbiosis. A positive functional screen of the entire S. meliloti genome for symbiotic genes was carried out using a modified in vivo expression technology. A group of genes and putative genes were found to be expressed in early stages of the symbiosis, and 23 of them were alfalfa root exudate inducible. These 23 genes were further separated into two groups based on their responses to apigenin, a known nodulation (nod) gene inducer. The group of six genes not inducible by apigenin included the lsrA gene, which is essential for the symbiosis, and the dgkA gene, which is involved in the synthesis of cyclic ß-1,2-glucan required for the S. meliloti-alfalfa symbiosis. In the group of 17 apigenin-inducible genes, most have not been previously characterized in S. meliloti, and none of them belongs to the nod gene family. The identification of this large group of alfalfa root exudate-inducible S. meliloti genes suggests that the interactions in the early stages of the S. meliloti and alfalfa symbiosis could be complex and that further characterization of these genes will lead to a better understanding of the symbiosis.

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* Corresponding author. Mailing address: Biological Sciences Department, Lehman College, The City University of New York, 250 Bedford Park Boulevard, West Bronx, NY 10468. Phone: (718) 960-7190. Fax: (718) 960-8236. E-mail: haiping.cheng{at}lehman.cuny.edu.

Supplemental material for this article may be found at http://www.aem.asm.org/.

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Applied and Environmental Microbiology, April 2006, p. 2738-2748, Vol. 72, No. 4
0099-2240/06/$08.00+0     doi:10.1128/AEM.72.4.2738-2748.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

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