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Journal of Bacteriology, September 2002, p. 4767-4774, Vol. 184, No. 17
0021-9193/02/$04.00+0     DOI: 10.1128/JB.184.17.4767-4774.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Hierarchy of Carbon Source Selection in Paracoccus pantotrophus: Strict Correlation between Reduction State of the Carbon Substrate and Aerobic Expression of the nap Operon

M. J. K. Ellington,1 K. K. Bhakoo,2 G. Sawers,3 D. J. Richardson,4 and S. J. Ferguson1*

Department of Biochemistry, University of Oxford, Oxford,1 Department of Molecular Microbiology, John Innes Centre, Norwich Research Park,2 Centre for Metalloprotein Spectroscopy and Biology, School of Biological Sciences, University of East Anglia, Norwich,3 MRC Clinical Sciences Centre, Imperial College of Science, Technology and Medicine, Hammersmith Campus, London, United Kingdom4

Received 5 March 2002/ Accepted 28 May 2002

Paracoccus pantotrophus can express a periplasmic nitrate reductase (Nap) during aerobic growth. A proposed role for this enzyme is the dissipation of excess redox energy during oxidative metabolism of reduced carbon substrates. To investigate the regulation of nap expression, a transcriptional fusion between the nap promoter region of P. pantotrophus and the lacZ gene was constructed. When this fusion was used, analyses showed that transcription from the nap promoter increases as the average reduction state of the carbon atoms increases. Thus, ß-galactosidase activities increase as the carbon source changes in the order succinate-acetate-butyrate. This result was obtained regardless of which of the three carbon sources was used for culture of the inoculum. If two carbon sources were presented together, the ß-galactosidase activity was always the same as it was when the least-reduced carbon source was added alone. This suggests that the regulation is dependent upon metabolism of the more-reduced carbon sources rather than just their presence in the medium. Analysis of culture medium by 1H nuclear magnetic resonance showed that for aerobic growth P. pantotrophus strictly selected its carbon source in the order succinate-acetate-butyrate. This was reflected by diauxic growth kinetics on medium containing mixed carbon substrates. The regulatory mechanism underpinning such a selection is unknown but is likely to be related to the mechanism which controls the transcription of the nap operon.

* Corresponding author. Mailing address: Department of Biochemistry, University of Oxford, South Parks Road, Oxford, United Kingdom. Phone: 44 (0) 1865 275240. Fax: 44 (0) 1865 275259. E-mail: stuart.ferguson{at}bioch.ox.ac.uk.

Journal of Bacteriology, September 2002, p. 4767-4774, Vol. 184, No. 17
0021-9193/02/$04.00+0     DOI: 10.1128/JB.184.17.4767-4774.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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