The rate-limiting step in melanogenesis is catalyzed by tyrosinase, a multifunctional enzyme encoded by the albino locus. We have previously reported that depletion of protein kinase C by long-term treatment of B16 mouse melanoma cells with phorbol dibutyrate (PDBu) prevented cell density-dependent melanogenesis. This was accompanied by a lack of induction of tyrosinase protein and mRNA. We report here the effect of PDBu on the functional activity of the mouse tyrosinase promoter by reporter gene assay and its effect on the binding of nuclear proteins from B16 cells to the "M-box" region of the mouse tyrosinase promoter. Short-term PDBu treatment of B16 cells transfected with a mouse tyrosinase promoter-luciferase construct resulted in increased reporter gene activity, while long-term PDBu treatment inhibited reporter gene activity. Using an oligonucleotide containing the M-box and its flanking residues in electrophoretic mobility shift assays, we found a density-dependent change in the pattern of DNA-protein complexes. One complex was found to be negatively regulated by long-term PDBu treatment. Competition experiments with various mutated oligonucleotides demonstrated that both the M-box and flanking residues are important for nuclear protein binding. The complex whose formation was inhibited by long-term PDBu treatment was shown to contain the basic helix-loop-helix leucine zipper protein microphthalmia-associated transcription factor (MITF). These results suggest that chronic PDBu treatment might inhibit tyrosinase expression (and subsequent melanogenesis) by affecting the amount or function of MITF.