Artificial chromosomes to explore and exploit biosynthetic capabilities of actinomycetes

Actinomycetes are an important source of biologically active compounds, like antibiotics, antitumor agents, and immuno-suppressors. Genome sequencing is revealing that this class of microorganisms has larger genomes relative to other bacteria, and uses a considerable fraction of its coding capacity (5–10%) for the production of mostly cryptic secondary metabolites. To access actinomycetes biosynthetic capabilities or to improve the pharmacokinetic properties and production yields of these chemically complex compounds, genetic manipulation of the producer strains can be performed. Heterologous expression in amenable hosts can be useful to exploit and to explore the genetic potential of actinomycetes and not cultivable but interesting bacteria. Artificial chromosomes that can be stably integrated into the Streptomyces genome were constructed and demonstrated to be effective for transferring entire biosynthetic gene clusters from intractable actinomycetes into more suitable hosts. In this review, the construction of several shuttle Escherichia coli-Streptomyces artificial chromosomes is discussed together with old and new strategies applied to improve heterologous production of secondary metabolites.