Heterologous expression of NRPS and PKS pathways in <i>Escherichia coli</i> : from bottlenecks to biosynthetic platforms

Microbial natural products (NPs) represent one of the most important sources of bioactive compounds in medicine, agriculture, and biotechnology. Non-ribosomal peptide synthetases (NRPSs) and polyketide synthases (PKSs) are large multidomain complexes that enable the efficient generation of highly diverse and complex NPs. Advances in genome sequencing and bioinformatics have revealed an enormous and largely untapped biosynthetic potential encoded in microbial genomes. However, the majority of NPs remain inaccessible due to a lack of genetic tools, silent biosynthetic gene clusters (BGCs) or the inability to culture the vast majority of microbes. Heterologous expression has therefore emerged as a key strategy to access and engineer NPs. Among available expression platforms, Escherichia coli is a pivotal host due to its rapid growth, genetic tractability, and unparalleled molecular toolbox. This review focuses on heterologous expression of NRPS/PKS pathways in E. coli, with particular emphasis on experimental optimisation strategies. Drawing on selected case studies, we highlight how systematic approaches-such as promoter refactoring, expression-rate tuning, chaperone or helper protein co-expression, metabolic engineering, precursor feeding, and cultivation optimisation-can transform initially unsuccessful systems into productive expression platforms. Collectively, these examples illustrate that apparent limitations in E. coli are engineering challenges rather than intrinsic barriers.