The effect of copper concentration of the culture medium on the production of copper-containing monoamine oxidase by Escherichia coli K-12 was studied. As previously reported, the addition of CuSO4 to the culture medium resulted in an increase in the total enzymatic activity. The added copper had been thought to enhance the activity of amine oxidases by affecting protein conformation. However, western blot analysis reveals that the addition of CuSO4 also results in an increase in the monoamine oxidase content of E. coli cells. Furthermore, northern blot analysis results indicate that an increase in monoamine oxidase activity also occurs at the transcriptional level. Increased transcription was observed in accordance with the addition of CuSO4 up to 50 mu M final concentration.