Members of the actinomycete genus Clavibacter are known to produce antimicrobial compounds, but so far none of these compounds has been purified and characterized. We have isolated an antimicrobial peptide, michiganin A, from the tomato pathogen Clavibacter michiganensis subsp. michiganensis, using ammonium sulfate precipitation followed by cation-exchange and reversed-phase chromatography steps. Upon chemical derivatization of putative dehydrated amino acids and lanthionine bridges by alkaline ethanethiol, Edman degradation yielded sequence information that proved to be sufficient for cloning of the gene by a genome-walking strategy. The mature unmodified peptide consists of 21 amino acids, SSSGWLCTLTIECGTIICACR. All of the threonine residues undergo dehydration, and three of them interact with cysteines via thioether bonds to form methyllanthionine bridges. Michiganin A resembles actagardine, a type B lantibiotic with a known three-dimensional structure, produced by Actinoplanes liguriae, which is a filamentous actinomycete. The DNA sequence of the gene showed that the michiganin A precursor contains an unusual putative signal peptide with no similarity to well-known secretion signals and only very limited similarity to the (only two) available leader peptides of other type B lantibiotics. Michiganin A inhibits the growth of Clavibacter michiganensis subsp. sepedonicus, the causal agent of ring rot of potatoes, with MICs in the low nanomolar range. Thus, michiganin A may have some potential in biological control of potato ring rot.