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Journal of Bacteriology, October 1998, p. 5070-5076, Vol. 180, No. 19
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Poly-beta -Hydroxybutyrate Turnover in Azorhizobium caulinodans Is Required for Growth and Affects nifA Expression

Karine Mandon,1,dagger Nathalie Michel-Reydellet,2 Sergio Encarnación,1 P. Alexandre Kaminski,2 Alfonso Leija,1 Miguel A. Cevallos,1 Claudine Elmerich,2,* and Jaime Mora1

Departamento de Ecología Molecular, Centro de Investigación sobre Fijación de Nitrógeno, Universidad Nacional Autónoma de México, Cuernavaca, Morelos, Mexico,1 and Unité de Physiologie Cellulaire and URA 1300 Centre National de Recherche Scientifique, Département des Biotechnologies, Institut Pasteur, 75724 Paris Cedex 15, France2

Received 24 February 1998/Accepted 6 August 1998

Azorhizobium caulinodans is able to fix nitrogen in the free-living state and in symbiosis with the tropical legume Sesbania rostrata. The bacteria accumulate poly-beta -hydroxybutyrate (PHB) under both conditions. The structural gene for PHB synthase, phbC, was inactivated by insertion of an interposon. The mutant strains obtained were devoid of PHB, impaired in their growth properties, totally devoid of nitrogenase activity ex planta (Nif-), and affected in nucleotide pools and induced Fix- nodules devoid of bacteria. The Nif- phenotype was the consequence of the lack of nifA transcription. Nitrogenase activity was partially restored to a phbC mutant by constitutive expression of the nifA gene. However, this constitutive nifA expression had no effect on the nucleotide content or on growth of the phbC mutant. It is suggested that PHB is required for maintaining the reducing power of the cell and therefore the bacterial growth. These observations also suggest a new control of nifA expression to adapt nitrogen fixation to the availability of carbon and reducing equivalents.

* Corresponding author. Mailing address: Unité de Physiologie Cellulaire, CNRS, URA 1300, Département des Biotechnologies, Institut Pasteur, 25 rue de Dr. Roux, 75724 Paris Cedex 15, France. Phone: 33 1 45 68 88 17. Fax: 33 1 45 68 87 90. E-mail: elmerich{at}pasteur.fr.

dagger Present address: Laboratoire de Biologie Végétale et Microbiologie, CNRS ERS 590, Université de Nice, 06108 Nice, France.

Journal of Bacteriology, October 1998, p. 5070-5076, Vol. 180, No. 19
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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