Abstract
Indole is long regarded as a typical N-heterocyclic aromatic pollutant in industrial and agricultural wastewater, and recently it has been identified as a versatile signaling molecule with wide environmental distributions. An exponentially growing number of researches have been reported on indole due to its significant roles in bacterial physiology, pathogenesis, animal behavior and human diseases. From the viewpoint of both environmental bioremediation and biological studies, the researches on metabolism and fates of indole are important to realize environmental treatment and illuminate its biological function. Indole can be produced from tryptophan by tryptophanase in many bacterial species. Meanwhile, various bacterial strains have obtained the ability to transform and degrade indole. The characteristics and pathways for indole degradation have been investigated for a century, and the functional genes for indole aerobic degradation have also been uncovered recently. Interestingly, many oxygenases have proven to be able to oxidize indole to indigo, and this historic and motivating case for biological applications has attracted intensive attention for decades. Herein, the bacteria, enzymes and pathways for indole production, biodegradation and biotransformation are systematically summarized, and the future researches on indole-microbe interactions are also prospected.
Highlights
Indole, which was firstly isolated from indigo reduction process, is a typical nitrogen heterocyclic aromatic compound widespread in our daily products and natural environment
Plant tryptophan synthase α (TSA) and tryptophan synthase β (TSB) showed high similarities with those in bacteria and catalyzed the conversion of indole-3-glycerol phosphate to tryptophan without indole release (Radwanski and Last, 1995)
Sequence analysis indicated that both indole-3-glycerol phosphate lyase (IGL) and BX1 should be derived from TSA
Summary
Indole, which was firstly isolated from indigo reduction process, is a typical nitrogen heterocyclic aromatic compound widespread in our daily products and natural environment. Indole ring is present in many alkaloids, phytohormones, plant flower oils, pigments and proteins (de Sá Alves et al, 2009; Arora et al, 2015). Since indole nucleus has a wide spectrum of biological activities, including anticancer, antiviral, antimicrobial, anti-inflammatory, antiHIV and antidiabetic activities, it is extensively used in the pharmaceutical industries (Gribble, 2010; Sharma et al, 2010). Relatively high concentration of indole has been found in coal tar and animal feces and sewage, constituting the major contributor to odor pollution (Yamamoto et al, 1991; Mackie et al, 1998)
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