Anaerobic degradation of p-ethylphenol by Aromatoleum aromaticum strain EbN1:: Pathway, regulation, and involved proteins

The denitrifying Aromatoleum aromaticum strain EbN1 was demonstrated to utilize p-ethylphenol under anoxic conditions and was suggested to employ a degradation pathway which is reminiscent of known anaerobic ethylbenzene degradation in the same bacterium: initial hydroxylation ofp-ethylphenol to 1-(4-hydroxyphenyl)ethanol followed by dehydrogenation to p-hydroxyacetophenone. Possibly, subsequent carboxylation and thiolytic cleavage yield p-hydroxybenzoyi-coenzyme A (CoA), which is channeled into the central benzoyl-CoA pathway. Substrate-specific formation of three of the four proposed intermediates was confirmed by gas chromatographic-mass spectrometric analysis and also by applying deuterated p-ethylphenol. Proteins suggested to be involved in this degradation pathway are encoded in a single large operon-like structure (-15 kb). Among them are ap-cresol methy1hydroxylase-like protein (PchCF), two predicted alcohol dehydrogenases (ChnA and EbA309), a biotin-dependent carboxylase (XccABQ, and a thiolase (TioL). Proteomic analysis (two-dimensional difference gel electrophoresis) revealed their specific and coordinated upregulation in cells adapted to anaerobic growth with p-ethylphenol and p-hydroxyacetophenone (e.g., PchF up to 29-fold). Coregulated proteins of' currently unknown function (e.g., EbA329) are possibly involved in p-ethylphenol- and p-hydroxyacetophenone-specific solvent stress responses and related to other aromatic solvent-induced proteins of strain EbN1.