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Applied and Environmental Microbiology, September 2006, p. 6070-6078, Vol. 72, No. 9
0099-2240/06/$08.00+0     doi:10.1128/AEM.00483-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

An Oxidoreductase Is Involved in Cercosporin Degradation by the Bacterium Xanthomonas campestris pv. zinniae

Tanya V. Taylor,¹ Thomas K. Mitchell,¹ and Margaret E. Daub^2*

Departments of Plant Pathology,¹ Botany, North Carolina State University, Raleigh, North Carolina 27695²

Received 28 February 2006/ Accepted 5 July 2006

The polyketide toxin cercosporin plays a key role in pathogenesis by fungal species of the genus Cercospora. The bacterium Xanthomonas campestris pv. zinniae is able to rapidly degrade this toxin. Growth of X. campestris pv. zinniae strains in cercosporin-containing medium leads to the breakdown of cercosporin and to the formation of xanosporic acid, a nontoxic breakdown product. Five non-cercosporin-degrading mutants of a strain that rapidly degrades cercosporin (XCZ-3) were generated by ethyl methanesulfonate mutagenesis and were then transformed with a genomic library from the wild-type strain. All five mutants were complemented with the same genomic clone, which encoded a putative transcriptional regulator and an oxidoreductase. Simultaneous expression of these two genes was necessary to complement the mutant phenotype. Sequence analysis of the mutants showed that all five mutants had point mutations in the oxidoreductase gene and no mutations in the regulator. Quantitative reverse transcription-PCR (RT-PCR) showed that the expression of both of these genes in the wild-type strain is upregulated after exposure to cercosporin. Both the oxidoreductase and transcriptional regulator genes were transformed into three non-cercosporin-degrading bacteria to determine if they are sufficient for cercosporin degradation. Quantitative RT-PCR analysis confirmed that the oxidoreductase was expressed in all transconjugants. However, none of the transconjugants were able to degrade cercosporin, suggesting that additional factors are required for cercosporin degradation. Further study of cercosporin degradation in X. campestris pv. zinniae may allow for the engineering of Cercospora-resistant plants by using a suite of genes.

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* Corresponding author. Mailing address: Department of Botany, 2214 Gardner Hall, North Carolina State University, Raleigh, NC 27695-7616. Phone: (919) 513-3807. Fax: (919) 515-3436. E-mail: margaret_daub{at}ncsu.edu.

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Applied and Environmental Microbiology, September 2006, p. 6070-6078, Vol. 72, No. 9
0099-2240/06/$08.00+0     doi:10.1128/AEM.00483-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

This article has been cited by other articles:

• Chen, H.-Q., Lee, M.-H., Chung, K.-R. (2007). Functional characterization of three genes encoding putative oxidoreductases required for cercosporin toxin biosynthesis in the fungus Cercospora nicotianae. Microbiology 153: 2781-2790 [Abstract] [Full Text]