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J. Bacteriol. doi:10.1128/JB.00260-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Genes, enzymes and regulation of para-cresol metabolism in Geobacter metallireducens

Insitute for Biology II, University of Freiburg, Germany; Laboratoire de Spéctrometrie de Masse Bio-Organique, CNRS, ECPM, Université Louis Pasteur de Strasbourg, France; Institute of Biochemistry, University of Leipzig, Germany

^* To whom correspondence should be addressed. Email: boll{at}uni-leipzig.de.

	Abstract

In aerobic and facultatively anaerobic bacteria the degradation of p-cresol involves the initial hydroxylation to p-hydroxybenzyl alcohol by water catalyzed by the soluble, periplasmatic flavocytochrome p-cresol methylhydroxylase (PCMH, ₂₂-composition). In denitrifying bacteria the further metabolism proceeds via oxidation to p-hydroxybenzoate, the formation of p-hydroxybenzoyl-CoA, and the subsequent dehydroxylation of the latter to benzoyl-CoA by reduction. In contrast, the strictly anaerobic Desulfobacterium cetonicum degrades p-cresol by addition to fumarate yielding p-hydroxybenzylsuccinate. In this work, in vitro enzyme activity measurements revealed that the obligately anaerobic Geobacer metallireducens uses the p-cresol degradation pathway of denitrifying bacteria. Surprisingly PCMH, which is supposed to catalyze both p-cresol hydroxylation and p-hydroxybenzyl alcohol oxidation to the corresponding aldehyde, was located in the membrane fraction. The -subunit of the enzyme was present in two isoforms suggesting an '₂-composition. We propose that the unusual asymmetric architecture and the membrane association of PCMH might be important for alternative electron transfer routes to either cytochrome c (in case of p-cresol oxidation) or to menaquinone (in case of p-hydroxybenzyl alcohol oxidation). Unusal properties of further enzymes of p-cresol metabolism, p-hydroxybenzoate-CoA ligase and p-hydroxybenzoyl-CoA reductase were identified and are discussed. A proteomic approach identified a gene cluster comprising most of the putative structural genes for enzymes involved in p-cresol metabolism (pcm-genes). Reverse transcription PCR studies revealed a different regulation of transcription of pcm-genes and the corresponding enzyme activities suggesting the presence of posttranscriptional regulatory elements.