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J Bacteriol, June 1998, p. 3253-3256, Vol. 180, No. 12
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

The FixK2 Protein Is Involved in Regulation of Symbiotic Hydrogenase Expression in Bradyrhizobium japonicum

Meredith C. Durmowicz and Robert J. Maier*

Department of Biology, Johns Hopkins University, Baltimore, Maryland 21218

Received 9 February 1998/Accepted 21 April 1998

The roles of the nitrogen fixation regulatory proteins NifA, FixK1, and FixK2 in the symbiotic regulation of hydrogenase structural gene expression in Bradyrhizobium japonicum have been investigated. Bacteroids from FixJ and FixK2 mutants have little or no hydrogenase activity, and extracts from these mutant bacteroids contain no hydrogenase protein. Bacteroids from a FixK1 mutant exhibit wild-type levels of hydrogenase activity. In beta -galactosidase transcriptional assays with NifA and FixK2 expression plasmids, the FixK2 protein induces transcription from the hup promoter to levels similar to those induced by HoxA, the transcriptional activator of free-living hydrogenase expression. The NifA protein does not activate transcription at the hydrogenase promoter. Therefore, FixK2 is involved in the transcriptional activation of symbiotic hydrogenase expression. By using beta -galactosidase transcriptional fusion constructs containing successive truncations of the hup promoter, the region of the hup promoter required for regulation by FixK2 was determined to be between 29 and 44 bp upstream of the transcription start site.

* Corresponding author. Mailing address: Department of Biology, Johns Hopkins University, Baltimore, MD 21218. Phone: (410) 516-7218. Fax: (410) 516-5213. E-mail: Maier_rj{at}jhuvms.hcf.jhu.edu.

J Bacteriol, June 1998, p. 3253-3256, Vol. 180, No. 12
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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