Microcin J25 (MccJ25) is the single representative of the immunity group J of the microcin group of peptide antibiotics produced by Enterobacteriaceae. It induces bacterial filamentation in susceptible cells in a non-SOSdependent pathway [R. A. Salomon and R. Farias (1992) J. Bacteriol. 174, 7428±7435]. MccJ25 was purified to homogeneity from the growth medium of a microcin-overproducing Escherichia coli strain by reverse-phase HPLC. Based on amino acid composition and absolute configuration determination, liquid secondary ion and electrospray mass spectrometry, extensive two-dimensional NMR, enzymatic and chemical degradations studies, the structure of MccJ25 was elucidated as a 21-residue peptide, cyclo(-Val 1 -Gly-Ile-Gly-Thr-Pro-Ile-Ser-Phe-Tyr-Gly-Gly-Gly-AlaGly-His-Val-Pro-Glu-Tyr-Phe 21 -). Although MccJ25 showed high resistance to most of endoproteases, linearization by thermolysin occurred from cleavage at the Phe 21 -Val 1 bond and led to a single peptide, MccJ25-L. While MccJ25 exhibited remarkable antibiotic activity towards Salmonella newport and several E. coli strains (minimal inhibitory concentrations ranging between 0.01 and 0.2 mg´mL ±1 ), the thermolysin-linearized microcin showed a dramatic decrease of the activity, indicating that the cyclic structure is essential for the MccJ25 biological properties. As MccJ25 is ribosomally synthesized as a larger peptide precursor endowed with an N-terminal extremity, the present study shows that removal of this extension 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.Keywords: antibiotic peptide; microcin; NMR.Microcins form a miscellaneous group of low molecular mass peptide antibiotics produced by diverse strains of Enterobacteriaceae, mostly Escherichia coli. They are mainly active against bacterial genera or species phylogenetically related to the producing strains [1,2]. Wild-type bacterial strains producing microcins are resistant to the microcin they produce, i.e. they are self-immune. This feature has been used to classify them into immunity groups [2], the number of which is growing [3,4]. Underlying this classification is the assumption that each immunity group represents a unique structure with an unique mode of action. Microcins, which are supposed to play a part in the stability of intestinal ecosystem [1,2,5,6], could be used as therapeutic agents or for the design of new useful drugs by means of DNA and protein engineering. To develop this research in a rational way, the knowledge of their chemical structures and of the enzymes involved in the biosyntheses appears to be required.Beside ColV, formerly classified as a colicin [7], the other best known microcins are MccB17 and MccC7. Their structures, modes of action and genes involved in their biosynthesis, export and immunity have been described. These microcins are plasmidencoded, ribosomally synthesized small peptides, whose production is induced when cells ...

No abstract