Anaerobic degradation of phenol and aniline by the sulfate-reducing bacterium Desulfatiglans anilini

Abstract

The anaerobic degradation pathways of aniline and phenol by sulfate-reducing bacteria are not yet fully understood. In this thesis, the sulfate-reducing bacterium Desulfatiglans anilini, which can grow with aniline or phenol as carbon and electron source, was investigated. In particular, this study is focused on the metabolic reactions and enzymes involved in anaerobic degradation of aniline or phenol by D. anilini and improvement of the growth of D. anilini with aniline. The study on phenol degradation in D. anilini describes the genes coding for the enzymes responsible for the anaerobic conversion of phenol to benzoyl-CoA which were identified by analyzing the genome of D. anilini, comparing the transcription of putative genes in phenol-grown or benzoate-grown cells by RT-PCR and comparing the translation of putative genes in phenolgrown or benzoate-grown cells by total proteome analysis. Further evidence suggests that phenylphosphate is an intermediate of phenol degradation in D. anilini as shown in in-vitro enzyme assays of phenylphosphate synthase. The results obtained revealed that phenol is most likely phosphorylated to phenylphosphate, followed by carboxylation to 4-hydroxybenzoate, which is analogous to the well-known phenol degradation pathway in the nitrate-reducing bacterium Thauera aromatica. To investigate the pathway of anaerobic aniline degradation by D. anilini, improving its growth is essential for further studies. A potential carboxylation activity was measured with carbon dioxide (CO2) as the co-substrate for activating aniline in the presence of ATP with cell-free extracts of D. anilini. Hydrogen sulfide, which is a product of sulfate reduction in sulfate-reducing bacterial cultures, was found to inhibit the enzymatic activity of the potential carboxylase during aniline activation and the phenylphosphate synthase during phenol activation. Hydrogen sulfide was also found to inhibit not only enzyme activities, but also growth of D. anilini when grown with aniline or phenol. A sulfide-consuming phototrophic bacterium Thiocapsa roseopersicina was co-cultured with D. anilini in a co-cultivation device to continuously remove hydrogen sulfide from the D. anilini culture. The doubling time of D. anilini was 15 days in the co-cultivation device, as compared to 26 days in the absence of a sulfide-oxidizing partner. The proteins specifically induced with aniline or 4-aminobenzoate were identified by comparing the total proteomes of D. anilini after growth with aniline, 4-aminobenzoate, benzoate or phenol. The genes coding for aniline-induced proteins are located in one single gene cluster (locus tag from H567DRAFT_03866 to H567DRAFT_03876) in the genome of D. anilini, including the genes coding for pyruvate water dikinase (H567DRAFT_03868), phosphoenolpyruvate synthase/pyruvate phosphate dikinase (H567DRAFT_03871) and phenylphosphate carboxylase beta subunit (H567DRAFT_03872). These three enzymes and phenylphosphate carboxylase beta subunit (H567DRAFT_02059) were successfully cloned and overexpressed in E. coli. The enzyme assays with recombinant proteins demonstrated that phenylphosphoamidate is carboxylated to 4aminobenzoate by enzymes encoded by the genes H567DRAFT_03872 and H567DRAFT_02059. Aniline is activated to 4-aminobenzoate by a combined reaction of the enzymes H567DRAFT_03868, H567D RAFT_03871, H567DRAFT_03872 and H567DRAFT_02059. We conclude that aniline is initially phosphorylated to phenylphosphoamidate by pyruvate water dikinase (H567DRAFT_03868) and phosphoenolpyruvate synthase/pyruvate phosphate dikinase (H567D RAFT_03871), and phenylphosphoamidate is subsequently carboxylated to 4aminobenzoate by phenylphosphate carboxylase beta subunit (H567DRAFT_03872 and H567DRAFT_02059).