Factors affecting phaeomelanin production by a melanin-producing (mel) mutant of Vibrio cholerae

In a previous study we isolated melanin-producing (mel) mutants of Vibrio cholerae and demonstrated that production of melanin during growth on solid media was stimulated by L-tyrosine and L-cysteine. In the studies reported here we analyzed factors that affected melanin production in liquid media and determined the abilities of radioactively labeled amino acids to serve as precursors for the formation of melanin by V. cholerae. Radioactivity from L-cysteine and from L-tyrosine was preferentially incorporated into partially purified melanin, providing further evidence for production of phaeomelanin by V. cholerae. Cuprous ions stimulated production of melanin by V. cholerae in defined liquid medium, but melanin formation was inhibited by high concentrations of L-cysteine or thiouracil. The inhibition of melanin formation by sulfhydryl compounds is most likely due to their ability to bind copper ions that are essential for tyrosinase activity.