5.1.3. Application of *Escherichia coli* as Heterologous Host

As the earliest heterologous host, *Escherichia coli* has been widely used in genetic engineering, metabolic engineering, and other fields. Because of its advantages, such as simple cultivable, rapid growth, high transformation efficiency, and so on, *E. coli* has a good effect on the heterologous expression of BGCs from prokaryotes. However, it has similar shortcomings with *S. cerevisiae*, lacking the introns splicing of eukaryotes and the posttranslational modification process, (glycosylation, phosphorylation et al.), which restrict the heterologous expression of the fungal biosynthetic gene cluster. It is generally used to heterologously express a single gene of BGC and is combined with in vitro enzyme experiments to characterize the protein function. The key genes in the BGCs of nanangelenin A [80], diorcinol [79], and asperaculin A [57], derived from *Aspergillus*, fumiquinazoline [81], and brevianamide A [73], derived from *Penicillium*, and trichobrasilenol, derived from *T. atroviride* [70], were heterogeneously expressed in *E. coli.*, combined with in vitro catalysis and other experiments, which clarified the functions of a series of key enzymes in the biosynthesis pathway.
