Characterization of mutants defective in melanogenesis and a gene for tyrosinase of Streptomyces griseus

Nitrosoguanidine-induced melanin-negative mutants of Streptomyces griseus were classified according to their ability to produce streptomycin and A-factor, and to form aerial mycelium. A large proportion of the mutants showed deficiency in either antibiotic production and morphological development or both, suggesting close regulatory correlation between melanogenesis and morphological and physiological differentiation. The tyrosinase-encoding met operon of S. griseus was cloned and examined for its role in melanogenesis of this organism. As in other Streptomyces homologues, the operon consisted of two open reading frames, melC1 encoding the putative cofactor and melC2 encoding the tyrosinase. Regardless of the distinct sequence similarity, introduction of the operon on plasmids failed to confer melanin production in the melanin-negative mutants, and the disruption of melC2 barely affected the melanin productivity, which indicated the presence of another enzyme involved in the melanogenesis in S. griseus. On the other hand, mel on a high-copy-number plasmid caused precocious aerial mycelium formation in Streptomyces lividans TK21 suggesting a stimulatory role of tyrosinase in morphological development.