Ustilago maydis Yeast Mutant Produces Cytosolic Melanin by Tyrosine-Tyrosinase Activity with Stain Biosorption Capability

Ustilago maydis is a biotrophic basidiomycete fungus that infects corn plants and works as an excellent phytopathogen model, facilitating numerous genetic transformations for studying the mechanisms of plant infection. A random mutation event in the mutant strains designed to investigate the physiological significance of two plasma membrane proton-ATPases in this model resulted in a pigmented phenotype strain. For this study, the FB2 strain and the ΔPMA1 mutant were chosen to assess the pigment, which was confirmed as melanin through thin-layer chromatography, UV, and IR spectrophotometry. The melanin was observed to accumulate in the cytosol, as evident from scanning and transmission electron microscopy, and did not interfere with normal cell growth in yeast extract peptone dextrose media or minimal media. Notably, the mutant exhibited a 25% higher melanin yield compared to wild-type cells. To analyze the melanin synthesis, the tyrosinase activity was measured in a phosphate buffer at pH 6.5. The enzyme demonstrated greater activity with tyrosine as a substrate than with L-3,4 dihydroxyphenylalanine, maintaining the same trend in ion preference. Both FB2 and ΔPMA1 mutant cells were subjected to biosorption experiments, revealing that the mutants with an excess of cytosolic melanin were capable of removing at least 50 ppm of methylene blue. In conclusion, U. maydis can accumulate melanin in the cytosol without adverse physiological effects and this presents biotechnological potential for dye removal.