A plant-specific biogenic amine, serotonin, was produced by heterologous expression of two key biosynthetic genes, tryptophan decarboxylase (TDC) and tryptamine 5-hydroxylase (T5H), in Escherichia coli. The native T5H, a cytochrome P450 enzyme, was unable to be functionally expressed in E. coli. Through a series of N-terminal deletions or additions of tagging proteins, we generated a functional T5H enzyme construct (GSTa dagger 37T5H) in which glutathione S transferase (GST) was translationally fused with the N-terminal 37 amino acid deleted T5H. Dual expression of GSTa dagger 37T5H and TDC using a pCOLADuet-1 E. coli vector produced serotonin at concentrations of approximately 24 mg l(-1) in the culture medium and 4 mg l(-1) in the cells. An optimum temperature of approximately 20A degrees C was required to achieve peak serotonin production in E. coli because the low induction temperature gave rise to the highest soluble expression of GSTa dagger 37T5H.