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Journal of Bacteriology, May 1999, p. 2745-2751, Vol. 181, No. 9
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Role of XDHC in Molybdenum Cofactor Insertion into Xanthine Dehydrogenase of Rhodobacter capsulatus

Silke Leimkühlerdagger and Werner Klipp*

Lehrstuhl für Biologie der Mikroorganismen, Fakultät für Biologie, Ruhr-Universität Bochum, D-44780 Bochum, Germany

Received 28 December 1998/Accepted 17 February 1999

Rhodobacter capsulatus xanthine dehydrogenase (XDH) is composed of two subunits, XDHA and XDHB. Immediately downstream of xdhB, a third gene was identified, designated xdhC, which is cotranscribed with xdhAB. Interposon mutagenesis revealed that the xdhC gene product is required for XDH activity. However, XDHC is not a subunit of active XDH, which forms an alpha 2beta 2 heterotetramer in R. capsulatus. It was shown that XDHC neither is a transcriptional regulator for xdh gene expression nor influences XDH stability. To analyze the function of XDHC for XDH in R. capsulatus, inactive XDH was purified from an xdhC mutant strain. Analysis of the molybdenum cofactor content of this enzyme demonstrated that in the absence of XDHC, no molybdopterin cofactor MPT is present in the XDHAB tetramer. In contrast, absorption spectra of inactive XDH isolated from the xdhC mutant revealed the presence of iron-sulfur clusters and flavin adenine dinucleotide, demonstrating that XDHC is not required for the insertion of these cofactors. The absence of MPT from XDH isolated from an xdhC mutant indicates that XDHC either acts as a specific MPT insertase or might be a specific chaperone facilitating the insertion of MPT and/or folding of XDH during or after cofactor insertion.

* Corresponding author. Mailing address: Lehrstuhl für Biologie der Mikroorganismen, Fakultät für Biologie, Ruhr-Universität Bochum, D-44780 Bochum, Germany. Phone: 49-234-700-3100. Fax: 49-234-7094-620. E-mail: werner.klipp{at}ruhr-uni-bochum.de.

dagger Present address: Department of Biochemistry, Duke University Medical Center, Durham, NC 27710.

Journal of Bacteriology, May 1999, p. 2745-2751, Vol. 181, No. 9
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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