Many bioactive compounds, including clinically important antibiotics such as erythromycin, are synthesized by microorganisms that often are only poorly accessible by molecular genetic techniques. Thus, genetic engineering of these biosynthetic pathways requires substantial efforts as basic protocols for gene transfer, DNA isolation, and mutagenesis have to be individually developed for each producing strain. Therefore, the expression of complete biosynthetic pathways in heterologous hosts is a promising way to engineer and improve such compounds. Heterologous expression of small gene clusters encoding biosynthesis of aromatic polyketides has already resulted in good yields (see Lopez et al., 2010) and allowed the manipulation of the pathways by standard methodology. Using this approach, derivatives of the natural compounds could be generated in a much easier way, when compared with engineering the original producing strains. Nevertheless, these studies mostly used closely related expression hosts.