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Journal of Bacteriology, July 2005, p. 4562-4572, Vol. 187, No. 13
0021-9193/05/$08.00+0     doi:10.1128/JB.187.13.4562-4572.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Two New Sinorhizobium meliloti LysR-Type Transcriptional Regulators Required for Nodulation

Biological Sciences Department, Lehman College, The City University of New York, Bronx, New York 10468,¹ Laboratory of Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institute of Biological Sciences, Chinese Academy of Science, Shanghai 200032, China,² Institut für Genomforschung, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany,³ Lehrstuhl fur Genetik, Fakultat fur Biologie, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany⁴

Received 25 June 2003/ Accepted 11 March 2005

The establishment of an effective nitrogen-fixing symbiosis between Sinorhizobium meliloti and its legume host alfalfa (Medicago sativa) depends on the timely expression of nodulation genes that are controlled by LysR-type regulators. Ninety putative genes coding for LysR-type transcriptional regulators were identified in the recently sequenced S. meliloti genome. All 90 putative lysR genes were mutagenized using plasmid insertions as a first step toward determining their roles in symbiosis. Two new LysR-type symbiosis regulator genes, lsrA and lsrB, were identified in the screening. Both the lsrA and lsrB genes are expressed in free-living S. meliloti cells, but they are not required for cell growth. An lsrA1 mutant was defective in symbiosis and elicited only white nodules that exhibited no nitrogenase activity. Cells of the lsrA1 mutant were recovered from the white nodules, suggesting that the lsrA1 mutant was blocked early in nodulation. An lsrB1 mutant was deficient in symbiosis and elicited a mixture of pink and white nodules on alfalfa plants. These plants exhibited lower overall nitrogenase activity than plants inoculated with the wild-type strain, which is consistent with the fact that most of the alfalfa plants inoculated with the lsrB1 mutant were short and yellow. Cells of the lsrB1 mutant were recovered from both pink and white nodules, suggesting that lsrB1 mutants could be blocked at multiple points during nodulation. The identification of two new LysR-type symbiosis transcriptional regulators provides two new avenues for understanding the complex S. meliloti-alfalfa interactions which occur during symbiosis.

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