AMINOCOUMARINS: MUTASYNTHESIS, CHEMOENZYMATIC SYNTHESIS, AND METABOLIC ENGINEERING

The aminocoumarin antibiotics novobiocin, clorobiocin and coumermycin A, are formed by different Streptomyces strains and are potent inhibitors of bacterial gyrase. Their biosynthetic gene clusters have been analyzed in detail by genetic and biochemical investigations. Heterologous expression of these gene clusters by site-specific integration into the genome of the fully sequenced host Streptomyces coelicolor A3(2) readily results in an accumulation of the antibiotics in yields similar to the wildtype strains. In recent years, the aminocoumarins have developed into a model system for the generation of new antibiotics by genetic methods. Prior to heterologous expression in S. coelicolor, cosmids containing the complete biosynthetic clusters can be manipulated in Escherichia coli by lambda. RED-mediated recombination, creating single or multiple gene replacements or gene deletions. Thereby, mutant strains are generated which are blocked in the synthesis of certain intermediates or in specific tailoring reactions. For instance, mutasynthetic experiments can subsequently be carried out to generate aminocoumarin antibiotics that contain modified acyl moieties attached to the aminocoumarin core, and chemoenzymatic synthesis can be employed for the acylation of the deoxysugar moiety of structural analogues of the aminocoumarin antibiotics. Metabolic engineering-the combination of gene deletions and foreign gene expression via replicative expression vectors-can be used to generate further structural variants of these antibiotics. These methods can be combined, allowing the generation of a wide variety of new compounds. This chapter may provide general pointers for the use of genetic methods in the generation of new antibiotics.