ABSTRACT
The denitrifying bacterium Thauera aromatica strain AR-1 grows anaerobically with protocatechuate (3,4-dihydroxybenzoate (DHB)) as sole energy and carbon source. This bacterium harbors two distinct pathways for degradation of aromatic compounds, the benzoyl-coenzyme A (CoA) pathway for benzoate degradation and the hydroxyhydroquinone (HHQ) pathway for degradation of 3,5-DHB. In order to elucidate whether protocatechuate is degraded via the benzoyl-CoA or the HHQ pathway, induction experiments were carried out. Dense suspensions of cells grown on protocatechuate or benzoate readily degraded benzoate and protocatechuate but not 3,5-DHB. Dense suspensions of 3,5-DHB-grown cells degraded 3,4- and 3,5-DHB at similar rates, but benzoate was not degraded. 3,5-DHB hydroxylating activity was found only in cells grown with this substrate. HHQ dehydrogenase activity was found in extracts of cells grown with 3,5-DHB and at a low rate also in protocatechuate-grown cells, but not in extracts of cells grown with benzoate. Activities of protocatechuyl-CoA synthetase and protocatechuyl-CoA reductase leading to 3-hydroxybenzoyl-CoA were found in extracts of cells grown with protocatechuate. There was no repression of the HHQ pathway by the presence of protocatechuate, unlike by degradation of benzoate. We conclude that protocatechuate is not degraded via the HHQ pathway because there was no evidence of a hydroxylation reaction involved in this process. Instead, our results strongly suggest that protocatechuate is degraded via a pathway which connects to the benzoyl-CoA route of degradation.