The cyclic structure of microcin J25, a 21-residue peptide antibiotic fromEscherichia coli

Microcin J25 (MccJ25) is the single representative of the immunity group I of the microcin group of peptide antibiotics produced by Enterobacteriaceae . It induces bacterial filamentation in susceptible cells in a non-SOS-depe ndent pathway [R. A. Salomon and R. Farias (1992) J. Bacteriol. 174, 7428-7 435]. MccJ25 was purified to homogeneity from the growth medium of a microc in-overproducing Escherichia coli strain by reverse-phase HPLC. Based on am ino acid composition and absolute configuration determination, liquid secon dary ion and electrospray mass spectrometry, extensive two-dimensional NMR, enzymatic and chemical degradations studies, the structure of MccJ25 was e lucidated as a 21-residue peptide, cyclo(-Val(1)-Gly-Ile-Gly-Thr-Pro-Ile-Se r-Phe-Tyr-Gly-Gly-Gly-Ala-Gly-His-Val-Pro-Glu-Tyr-Phe(21)-). Although MccJ2 5 showed high resistance to most of endoproteases, linearization by thermol ysin occurred from cleavage at the Phe(21)-Val(1) bond and led to a single peptide, MccJ25-L. While MccJ25 exhibited remarkable antibiotic activity to wards Salmonella newport and several E. coli strains (minimal inhibitory co ncentrations ranging between 0.01 and 0.2 mu g.mL(-1)), the thermolysin-lin earized microcin showed a dramatic decrease of the activity, indicating tha t the cyclic structure is essential for the MccJ25 biological properties. A s MccJ25 is ribosomally synthesized as a larger peptide precursor endowed w ith an N-terminal extremity, the present study shows that removal of this e xtension and head-tail cyclization of the resulting propeptide are the only posttranslational modifications involved in the maturation of MccJ25, that appears as the first cyclic microcin.