We constructed a promoter-probe vector, designated pMX180, carrying a Streptomyces tyrosinase gene as a reporter gene. In the present study, Escherichia coli harboring the plasmid pMX180tac, generated by insertion of a tac promoter into pMX180, produced melanin mediated by tyrosinase, in the presence of antibiotic nucleic acid synthesis inhibitors. This suggests that pMX180tac is useful for visible detection of potent inhibitors of nucleic acid synthesis.