Abstract
The assembly-line synthases that produce bacterial polyketide natural products follow a modular paradigm in which each round of chain extension is catalysed by a different set or module of enzymes. Examples of deviation from this paradigm, in which a module catalyses either multiple extensions or none are of interest from both a mechanistic and an evolutionary viewpoint. We present evidence that in the biosynthesis of the 36-membered macrocyclic aminopolyol lactones (marginolactones) azalomycin and kanchanamycin, isolated respectively from Streptomyces malaysiensis DSM4137 and Streptomyces olivaceus Tü4018, the first extension module catalyses both the first and second cycles of polyketide chain extension. To confirm the integrity of the azl gene cluster, it was cloned intact on a bacterial artificial chromosome and transplanted into the heterologous host strain Streptomyces lividans, which does not possess the genes for marginolactone production. When furnished with 4-guanidinobutyramide, a specific precursor of the azalomycin starter unit, the recombinant S. lividans produced azalomycin, showing that the polyketide synthase genes in the sequenced cluster are sufficient to accomplish formation of the full-length polyketide chain. This provides strong support for module iteration in the azalomycin and kanchanamycin biosynthetic pathways. In contrast, re-sequencing of the gene cluster for biosynthesis of the polyketide β-lactone ebelactone in Streptomyces aburaviensis has shown that, contrary to a recently-published proposal, the ebelactone polyketide synthase faithfully follows the colinear modular paradigm.
Highlights
Bacterial modular Type I polyketide synthases (PKSs) are multienzymes that govern the biosynthesis of diverse complex polyketide natural products, including clinically useful antibiotics, immunosuppressants, and antitumor compounds
We have previously shown that the unusual starter unit in azalomycin biosynthesis, 4-guanidinobutanoyl-CoA, requires a dedicated three-enzyme precursor pathway: L-arginine is converted by arginine monooxygenase to 4-guanidinobutyramide, 4-guanidinobutyramide hydrolase converts this to 4-guanidinobutyric acid, and the acid is activated to 4-guanidinobutyl-CoA by 4-guanidinobutanoate:CoA ligase [35]
New exceptions to the so-called colinear rule are of great interest for our understanding of the paradigm of enzyme catalysis used by bacterial modular PKS multienzymes
Summary
Bacterial modular Type I polyketide synthases (PKSs) are multienzymes that govern the biosynthesis of diverse complex polyketide natural products, including clinically useful antibiotics, immunosuppressants, and antitumor compounds. They follow a remarkable assembly-line paradigm, in which each cycle of polyketide chain extension is accomplished by a differ-. This processive assembly-line operation, in which all intermediates remain covalently attached to the multienzyme, helps to explain the efficiency of the process It neatly explains how the diversity of naturally-occurring complex polyketides is generated by a common biosynthetic mechanism, and provides clues to the evolution of these multienzymes through duplication, capture, deletion, and rearrangement of modules or individual domains [5]. It has both prompted efforts to manipulate PKS domains and modules into novel combinations, as a route to obtaining novel non-natural polyketide products [6,7], and facilitated the discovery of new biosynthetic gene clusters using whole-genome sequence analysis [8,9]
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