Two distinct pathways for anaerobic degradation of aromatic compounds in the denitrifying bacterium Thauera aromatica strain AR-1

Denitrifying bacteria degrade many different aromatic compounds anaerobical ly via the well-described benzoyl-CoA pathway. We have shown recently that the denitrifiers Azoarcus anaerobius and Thaauera aromatica strain AR-I use a different pathway for anaerobic degradation of resorcinol (1,3-dihydroxy benzene) and 3,5-dihydroxybenzoate, respectively. Both substrates are conve rted to hydroxyhydroquinone (1,2,4-trihydroxybenzene). In the membrane frac tion of T. aromatica strain AR-1 cells grown with 3,5-dihydroxybenzoate, a hydroxyhydroquinone-dehydrogenating activity of 74 nmol min(-1)(mg protein) (-1) was found. This activity was significantly lower in benzoate-grown cel ls. Benzoate-grown cells were not induced for degradation of 3,5-dihydroxyb enzoate, and cells grown with 3,5-dihydroxybenzoate degraded benzoate only at a very low rate. With a substrate mixture of benzoate plus 3,5-dihydroxy benzoate, the cells showed diauxic growth. Benzoate was degraded first, whi le complete degradation of 3,5-dihydroxybenzoate occurred only after a long lag phase. The 3,5-dihydroxybenzoate-oxidizing and the hydroxyhydroquinone -dehydrogenating activities were fully induced only during 3,5-dihydroxyben zoate degradation Synthesis of benzoyl-CoA reductase appeared to be signifi cantly lower in 3,5-dihydroxybenzoate-grown cells as shown by immunoblottin g. These results confirm that T. aromatica strain AR-1 harbors, in addition to the benzoyl-CoA pathway, a second, mechanistically distinct pathway for anaerobic degradation of aromatic compounds. This pathway is inducible and subject to catabolite repression by benzoate.