Indole Biodegradation in Acinetobacter sp. Strain O153: Genetic and Biochemical Characterization

Abstract

Indole is a molecule of considerable biochemical significance, acting as both an interspecies signal molecule and a building block of biological elements. Bacterial indole degradation has been demonstrated for a number of cases; however, very little is known about genes and proteins involved in this process. This study reports the cloning and initial functional characterization of genes (<i>iif</i> and <i>ant</i> cluster) responsible for indole biodegradation in <i>Acinetobacter</i> sp. strain O153. The catabolic cascade was reconstituted <i>in vitro</i> with recombinant proteins, and each protein was assigned an enzymatic function. Degradation starts with oxidation, mediated by the IifC and IifD flavin-dependent two-component oxygenase system. Formation of indigo is prevented by IifB, and the final product, anthranilic acid, is formed by IifA, an enzyme which is both structurally and functionally comparable to cofactor-independent oxygenases. Moreover, the <i>iif</i> cluster was identified in the genomes of a wide range of bacteria, suggesting the potential of widespread Iif-mediated indole degradation. This work provides novel insights into the genetic background of microbial indole biodegradation.<b>IMPORTANCE</b> The key finding of this research is identification of the genes responsible for microbial biodegradation of indole, a toxic <i>N</i>-heterocyclic compound. A large amount of indole is present in urban wastewater and sewage sludge, creating a demand for an efficient and eco-friendly means to eliminate this pollutant. A common strategy of oxidizing indole to indigo has the major drawback of producing insoluble material. Genes and proteins of <i>Acinetobacter</i> sp. strain O153 (DSM 103907) reported here pave the way for effective and indigo-free indole removal. In addition, this work suggests possible novel means of indole-mediated bacterial interactions and provides the basis for future research on indole metabolism.