Melanin production as a visual indicator of conjugal transfer in Streptomyces

To visualize transfer of plasmid in Streptomyces during conjugation, we constructed a conjugative plasmid that harbored melC operon encoding an extracellular tyrosinase and placed it in Streptomyces hosts which were defective in expressing the operon. Hyphae of these donors were mixed with hyphae of a plasmidless recipient, which could express melC, and plated on a solid medium supplemented with tyrosine. After 8 to 9 h of incubation, melanin started to appear in the mating mixture, indicating that the plasmid had entered the recipient and started to synthesize tyrosinase, which in turn catalyzed the formation of melanin. This visual monitoring system allows quick demonstration of conjugal transfer without tedious genetic or biochemical procedure commonly used. It may be applied to most Streptomyces species and may also be used for monitoring chromosome transfer.