Knowledge-based design of bimodular and trimodular polyketide synthases based on domain and module swaps: a route to simple statin analogues

AbstractBackground: Polyketides are structurally diverse natural products that have a range of medically useful activities. Nonaromatic bacterial polyketides are synthesised on modular polyketide synthase (PKS) multienzymes, in which each cycle of chain extension requires a different ‘module’ of enzymatic activities. Attempts to design and construct modular PKSs that synthesise specified novel polyketides provide a particularly stringent test of our understanding of PKS structure and function.Results: We have constructed bimodular and trimodular PKSs based on DEBS1-TE, a derivative of the erythromycin PKS that contains only modules 1 and 2 and a thioesterase (TE), by substituting multiple domains with appropriate counterparts derived from the rapamycin PKS. Hybrid PKSs were obtained that synthesised the predicted target triketide lactones, which are simple analogues of cholesterol-lowering statins. In constructing intermodular fusions, whether between modules in the same or in different proteins, it was found advantageous to preserve intact the acyl carrier protein-ketosynthase (ACP-KS) didomain that spans the junction between successive modules.Conclusions: Relatively simple considerations govern the construction of functional hybrid PKSs. Fusion sites should be chosen either in the surface-accessible linker regions between enzymatic domains, as previously revealed, or just inside the conserved margins of domains. The interaction of an ACP domain with the adjacent KS domain, whether on the same polyketide or not, is of particular importance, both through conservation of appropriate protein-protein interactions, and through optimising molecular recognition of the altered polyketide chain in the key transfer of the acyl chain from the ACP of one module to the KS of the downstream module

Activity of the Novel Macrolide BAL19403 against Ribosomes from Erythromycin-Resistant Propionibacterium acnes▿

BAL19403 is a macrolide antibiotic from a novel structural class with potent activity against propionibacteria in vitro. The antibacterial spectrum of BAL19403 covers clinical isolates with mutations in the 2057 to 2059 region of 23S rRNA that confer resistance to erythromycin and clindamycin. The basis of this improved activity was investigated by ribosome binding assays and by a coupled transcription and translation assay. The latter was specifically developed for the use of ribosomes from Propionibacterium acnes. BAL19403 inhibited protein expression by ribosomes from erythromycin-sensitive and erythromycin-resistant P. acnes with similar potencies if the resistance was due to G2057A or A2058G mutations. BAL19403 showed a >10-fold higher activity than erythromycin against ribosomes from a strain with the erm(X) gene. Erm(X) confers high levels of macrolide and lincosamide resistance by dimethylation of A2058. Assays with such ribosomes showed that BAL19403 was potent enough to inhibit half of the total activity with a 50% inhibitory concentration very close to the value measured with erythromycin-sensitive ribosomes. We concluded from our data that the P. acnes strain with the erm(X) gene had a mixed population of ribosomes, with macrolide-sensitive and macrolide-resistant species