New Type of Antimicrobial Protein Produced by the Plant Pathogen Clavibacter michiganensis subsp. michiganensis

It has previously been shown that the tomato pathogenClavibacter michiganensis subsp.michiganensis secretes a 14-kDa protein,C. michiganensis subsp.michiganensis AMP-I (Cmm AMP-I), that inhibits growth ofClavibacter michiganensis subsp.sepedonicus , the causal agent of bacterial ring rot of potato. Using sequences obtained from tryptic fragments, we have identified the gene encodingCmm AMP-I and we have recombinantly produced the protein with an N-terminal intein tag. The gene sequence showed thatCmm AMP-I contains a typical N-terminal signal peptide for Sec-dependent secretion. The recombinant protein was highly active, with 50% growth inhibition (IC50 ) of approximately 10 pmol, but was not toxic to potato leaves or tubers.Cmm AMP-I does not resemble any known protein and thus represents a completely new type of bacteriocin. Due to its high antimicrobial activity and its very narrow inhibitory spectrum,Cmm AMP-1 may be of interest in combating potato ring rot disease.