Biosynthesis and functions of a melanoid pigment produced by species of the sporothrix complex in the presence of L-Tyrosine

Sporothrix schenckii is the etiological agent of sporotrichosis, the main subcutaneous mycosis in Latin America. Melanin is an important virulence factor of S. schenckii, which produces dihydroxynaphthalene melanin (DHN-melanin) in conidia and yeast cells. Additionally, L-dihydroxyphenylalanine (L-DOPA) can be used to enhance melanin production on these structures as well as on hyphae. Some fungi are able to synthesize another type of melanoid pigment, called pyomelanin, as a result of tyrosine catabolism. Since there is no information about tyrosine catabolism in Sporothrix spp., we cultured 73 strains, including representatives of newly described Sporothrix species of medical interest, such as S. brasiliensis, S. schenckii, and S. globosa, in minimal medium with tyrosine. All strains but one were able to produce a melanoid pigment with a negative charge in this culture medium after 9 days of incubation. An S. schenckii DHN-melanin mutant strain also produced pigment in the presence of tyrosine. Further analysis showed that pigment production occurs in both the filamentous and yeast phases, and pigment accumulates in supernatants during stationary-phase growth. Notably, sulcotrione inhibits pigment production. Melanin ghosts of wild-type and DHN mutant strains obtained when the fungus was cultured with tyrosine were similar to melanin ghosts yielded in the absence of the precursor, indicating that this melanin does not polymerize on the fungal cell wall. However, pyomelanin-producing fungal cells were more resistant to nitrogen-derived oxidants and to UV light. In conclusion, at least three species of the Sporothrix complex are able to produce pyomelanin in the presence of tyrosine, and this pigment might be involved in virulence.