Pathway engineering in secondary metabolite-producing actinomycetes.

Actinomycetes represent the microbial group richest in production of variable secondary metabolites. These mostly bioactive molecules are the end products of complex multistep biosynthetic pathways. Recent progress in the molecular genetics and biochemistry of the biosynthetic capacities of actinomycetes enables first attempts to redesign these pathways in a directed fashion. However, in contrast to several examples of designed biochemical improvement of primary metabolic processes in microorganisms, none of the products or strains derived from pathway engineering in actinomycetes discussed herein have reached pilot or production scale. The main reasons for this slow progress are the complicated pathways themselves, their complex regulation during the actinomycete cell cycle, and their uniqueness, as most pathways and products are specific for a strain rather than for a given species or larger taxonomic group. However, the modular use of a minimum of very similar enzymes and their conversion of similar intermediates to form the building blocks for the production of a maximum of divergent end products gives hope for the future application of these genetic models for the redesign of complex pathways for modified or new natural products. Several strategies that can be followed to reach this aim are discussed, mainly for the variable 6-deoxyhexose metabolism as an ubiquitously applicable example.