Biosynthesis and characterization of violacein, deoxyviolacein and oxyviolacein in heterologous host, and their antimicrobial activities

Violacein is a natural bisindole pigment produced by bacteria, which has attracted much attention recently for its antibacterial, antiviral, antitumor, and antioxidant activities. To better understand the bioactive properties of violacein derivatives, deoxyviolacein and oxyviolacein were biosynthesized by genetically engineered strains of Citrobacter freundii(pComvio) and C. freundii(pComvioΔd), respectively, using l-tryptophan or 5-OH-l-tryptophan as the precursor, respectively. The structure of oxyviolacein was confirmed based on HPLC–MS and NMR analysis. Deoxyviolacein and oxyviolacein were evaluated for their effectiveness against bacteria and phytopathogenic fungi. The two pigments exhibited strong activities against all Gram-positive bacteria tested except Streptococcus sp. Oxyviolacein showed a surprisingly high activity against Phytophthora capsici, a devastating pathogen of vegetable crops, and also showed strong antifungal activity against Fusarium oxysporum, Botrytis cinerea, and Verticillium dahliae at 0.5 mg mL−1. Deoxyviolacein showed strong antifungal activity against Rhizoctonia solani at concentrations exceeding 2 mg mL−1. The presence of a hydroxyl group appeared to be essential for their bioactivities. These findings provide important information for the further exploitation of violacein and its derivatives as fungicides.

► Violacein derivatives were synthesized by recombinant strains with 5-HTP as precursor.
► Violacein derivative was identified as oxyviolacein based on HPLC–MS and NMR analysis.
► Oxyviolacein and deoxyviolacein exhibited strong activities against G+ bacteria.
► Oxyviolacein displayed strong activity against Phytophthora capsici compared to violacein.