Proposed pathway of degradation of indigo carmine and its co-metabolism by white-rot fungus Cyathus bulleri

Abstract

The stimulation of growth by the addition of indigo carmine (5, 5 indigo disulfonic acid sodium salt, IC) was studied in the white-rot fungus Cyathus bulleri. C. bulleri was grown on a variety of solid medium and addition of IC (5000 mg/L) resulted in enhancement of growth, particularly on malt-extract agar. Stimulation of growth by IC was also observed in liquid cultures grown in potato dextrose broth. IC was degraded into anthranilic acid by oxidoreductases, particularly laccases, present in the culture broth of the fungus grown on wheat bran and orange peelings, which was confirmed by treatment of IC with purified laccase. Anthranilic acid was proposed to further enter into the tryptophan (TRP) biosynthetic pathway, as demonstrated by up-regulation of newly identified trpD, trpC and trp1 genes from this fungus. These genes encode anthranilate phosphoribosyl transferase, a multi-functional enzyme with phosphoribosyl anthranilate isomerase/other activities and tryptophan synthase respectively. No such increased transcription of these genes was observed in the absence of IC.