Pyralomicin 1a

The biosynthetic pathway leading to the cyclitol moiety of pyralomicin 1a (1) in Nonomuraea spiralis MI178-34F18 has been studied using a series of 2H-labeled potential precursors. The results demonstrate that 2-epi-5-epi-valiolone (7), a common precursor for acarbose (4) and validamycin A (5) biosynthesis, is an immediate precursor of pyralomicin 1a. 5-epi-Valiolone (8) was also incorporated into 1, albeit less efficiently than 7. Other potential intermediates, such as valiolone (9), valienone (10), valienol (11), 1-epi-valienol (12), 5-epi-valiolol (13), and 1-epi-5-epi-valiolol (14) were not incorporated into pyralomicin 1a. To explain this surprising observation, it is proposed that either 2-epi-5-epi-valiolone (7) is specifically activated (e.g., to its phosphate) and that the further transformations take place on activated intermediates (which can not be generated directly from their unactivated counterparts), or that the transformation of 7 into 1 involves a substrate-channeling mechanism in which enzyme-bound intermediates are directly transferred from one enzyme active site to the next in a multi-enzyme complex.

The biosynthetic gene cluster for the pyralomicin antibiotics has been cloned and sequenced from Nonomuraea spiralis IMC A-0156. The 41 kb gene cluster contains 27 ORFs predicted to encode all of the functions for pyralomicin biosynthesis. This includes nonribosomal peptide synthetases (NRPS) and polyketide synthases (PKS) required for the formation of the benzopyranopyrrole core unit, as well as a suite of tailoring enzymes (e.g., four halogenases, an O-methyltransferase, and an N-glycosyltransferase) necessary for further modifications of the core structure. The N-glycosyltransferase is predicted to transfer either glucose or a pseudosugar (cyclitol) to the aglycone. A gene cassette encoding C7-cyclitol biosynthetic enzymes was identified upstream of the benzopyranopyrrole-specific ORFs. Targeted disruption of the gene encoding the N-glycosyltransferase, prlH, abolished pyralomicin production, and recombinant expression of PrlA confirms the activity of this enzyme as a sugar phosphate cyclase involved in the formation of the C7-cyclitol moiety.

This review covers microbial secondary metabolites classified in the family of C7N aminocyclitols, a relatively new class of natural products that is increasingly gaining recognition due to their significant biomedical and agricultural uses. Their discovery and structure determinations, their biosynthetic origin, biological properties, chemical synthesis, as well as their further development for pharmaceutical uses are described. The literature from 1970 to July 2002 is reviewed, with 269 references cited.