Anaerobic degradation of 4‐methylbenzoate via a specific 4‐methylbenzoyl‐CoA pathway

Abstract

Summary The pathway for anaerobic degradation of 4‐methylbenzoate was studied in the denitrifying alphaproteobacterium Magnetospirillum sp. strain pMbN1. Adaptation studies with whole cells indicated substrate‐dependent induction of the capacity to degrade 4‐methylbenzoate. Differential protein profiling (2D‐DIGE) of 4‐methylbenzoate‐ in comparison with benzoate‐ or succinate‐adapted cells revealed the specific abundance increase of substrate‐specific protein sets. Their coding genes form distinct clusters on the genome, two of which were assigned to 4‐methylbenzoate and one to benzoate degradation. The predicted functions of the gene products agree with a specific 4‐methylbenzoyl‐CoA degradation pathway in addition to and analogous to the known anaerobic benzoyl‐CoA degradation pathway. In vitro benzoyl‐CoA and 4‐methylbenzoyl‐CoA reductase activities revealed the electron donor and ATP‐dependent formation of the corresponding conjugated cyclic dienoyl‐CoA/4‐methyl‐dienoyl‐CoA products. The 4‐methylbenzoyl‐CoA reductase activity was induced in the presence of 4‐methylbenzoate. In accordance, metabolite analysis of cultures grown with 4‐methylbenzoate tentatively identified 4‐methylcyclohex‐1,5‐diene‐1‐carboxylate. The 4‐methylbenzoate induced genes were assigned to code for the putative 4‐methylbenzoyl‐CoA reductase; their products display pronounced sequence disparity from the conventional class I benzoyl‐CoA reductase, which does not accept substituents at the para ‐position. Identification of 3‐methylglutarate together with the formation of specific proteins for ring cleavage and β‐oxidation in 4‐methylbenzoate‐adapted cells suggest conservation of the methyl group along the specific 4‐methylbenzoyl‐CoA degradation pathway.

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