In vivo processing and antibiotic activity of microcin B17 analogs with varying ring content and altered bisheterocyclic sites

Background: The Escherichia coli peptide antibiotic microcin B17 (MccB17) c ontains four oxazole and four thiazole rings, and inhibits DNA gyrase,The r ole of individual and tandem pairs of heterocycles in bioactivity has not b een determined previously. Results: The two tandem 4,2-bisheterocycles in MccB17 were varied by expres sion of MccB17 or mutants containing altered sequences at Gly39-Ser40-Cys41 or Gly54-Cys55-Ser56. A mixture of five-nine-ring MccB17 isoforms were sep arated and quantitated for antibiotic potency. Mutagenesis of the thiazole- oxazole pair significantly affected antibiotic activity compared with the u pstream oxazole-thiazole, which might stabilize partially cyclized intermed iates against proteolysis. Conclusions: Enzymatic heterocyclization in native MccB17 occurs distributi vely, Antibiotic activity correlates with the number of rings and is differ entially sensitive to both the location and the identity of the 4,2-tandem heterocycle pairs in MccB17. Such tandem heterocycles might be useful pharm acophores in combinatorial libraries.