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Applied and Environmental Microbiology, March 2004, p. 1287-1296, Vol. 70, No. 3
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.3.1287-1296.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Regulatory Role of Rhizobium etli CNPAF512 fnrN during Symbiosis

Martine Moris, Bruno Dombrecht, Chuanwu Xi, Jos Vanderleyden, and Jan Michiels*

Centre of Microbial and Plant Genetics, Katholieke Universiteit Leuven, B-3001 Heverlee, Belgium

Received 8 July 2003/ Accepted 19 November 2003

The Rhizobium etli CNPAF512 fnrN gene was identified in the fixABCX rpoN2 region. The corresponding protein contains the hallmark residues characteristic of proteins belonging to the class IB group of Fnr-related proteins. The expression of R. etli fnrN is highly induced under free-living microaerobic conditions and during symbiosis. This microaerobic and symbiotic induction of fnrN is not controlled by the sigma factor RpoN and the symbiotic regulator nifA or fixLJ, but it is due to positive autoregulation. Inoculation of Phaseolus vulgaris with an R. etli fnrN mutant strain resulted in a severe reduction in the bacteroid nitrogen fixation capacity compared to the wild-type capacity, confirming the importance of FnrN during symbiosis. The expression of the R. etli fixN, fixG, and arcA genes is strictly controlled by fnrN under free-living microaerobic conditions and in bacteroids during symbiosis with the host. However, there is an additional level of regulation of fixN and fixG under symbiotic conditions. A phylogenetic analysis of the available rhizobial FnrN and FixK proteins grouped the proteins in three different clusters.

* Corresponding author. Mailing address: Centre of Microbial and Plant Genetics, K.U. Leuven, Kasteelpark Arenberg 20, B-3001 Heverlee, Belgium. Phone: (32-16) 321631. Fax: (32-16) 321963. E-mail: jan.michiels{at}agr.kuleuven.ac.be.

Applied and Environmental Microbiology, March 2004, p. 1287-1296, Vol. 70, No. 3
0099-2240/04/$08.00+0     DOI: 10.1128/AEM.70.3.1287-1296.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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