Anaerobic benzene oxidation in involves activation by methylation and is regulated by the transition state regulator AbrB.

UNLABELLED

Benzene is a widespread groundwater contaminant that persists under anoxic conditions. The aim of this study was to more accurately investigate anaerobic microbial degradation pathways to predict benzene fate and transport. Preliminary genomic analysis of strain FRC-32, isolated from contaminated groundwater, revealed the presence of putative aromatic-degrading genes. was subsequently shown to conserve energy for growth on benzene as the sole electron donor and fumarate or nitrate as the electron acceptor. The gene, encoding for 3-hydroxybenzylsuccinate synthase (Hbs), a homolog of the radical-forming, toluene-activating benzylsuccinate synthase (Bss), was upregulated during benzene oxidation in , while the gene was upregulated during toluene oxidation. Addition of benzene to the whole-cell lysate resulted in toluene formation, indicating that methylation of benzene was occurring. Complementation of σ (deficient) transformed with the operon restored its ability to grow in the presence of toluene, revealing to be regulated by σ. Binding sites for σ and the transition state regulator AbrB were identified in the promoter region of the σ-encoding gene and binding was confirmed. Induced expression of during benzene and toluene degradation caused cultures to transition to the death phase. Our results suggested that can anaerobically oxidize benzene by methylation, which is regulated by σ and AbrB. Our findings further indicated that the benzene, toluene, and benzoate degradation pathways converge into a single metabolic pathway, representing a uniquely efficient approach to anaerobic aromatic degradation in .

IMPORTANCE

The contamination of anaerobic subsurface environments including groundwater with toxic aromatic hydrocarbons, specifically benzene, toluene, ethylbenzene, and xylene, has become a global issue. Subsurface groundwater is largely anoxic, and further study is needed to understand the natural attenuation of these compounds. This study elucidated a metabolic pathway utilized by the bacterium capable of anaerobically degrading the recalcitrant molecule benzene using a unique activation mechanism involving methylation. The identification of aromatic-degrading genes and AbrB as a regulator of the anaerobic benzene and toluene degradation pathways provides insights into the mechanisms employed by to modulate metabolic pathways as necessary to thrive in anoxic contaminated groundwater. Our findings contribute to the understanding of novel anaerobic benzene degradation pathways that could potentially be harnessed to develop improved strategies for bioremediation of groundwater contaminants.