The melanin operon of Streptomyces antibioticus: expression and use as a marker in Gram-negative bacteria

The melC operon of Streptomyces antibioticus contains two genes, melC1 and melC2, necessary for the production of melanin pigment. We transferred the coding sequence of melC1 and melC2 to Escherichia coli plasmid pMTL23 such that its transcription was under the control of the lac promoter and melC1 was translationally fused to the lacZα fragment. E. coli cultures containing this plasmid, pIF413, produced melanin after overnight incubation on 2YT agar supplemented with 0.1 mM CuCl₂, 0.36 mM IPTG (or 0.2% lactose), and 2 mM tyrosine. Erwina carotovora could also be transformed by pIF413 to produce melanin. Two shuttle vectors were constructed: pLUS415 for E. coli and Streptomyces, and pLAF413 for E. coli and Xanthomonas campestris. These vectors confer melanin pigmentation in all the hosts that harbor them. The melC sequence provides the vectors with a convenient cloning marker for insertional or replacement inactivation.