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Type IIa bacteriocins, which are isolated from lactic acid bacteria that are useful for food preservation, are potent antimicrobial peptides with considerable potential as therapeutic agents for gastrointestinal infections in mammals. They are ribosomally synthesized as precursors with an N-terminal leader, typically 18-24 amino acid residues in length, which is cleaved during export from the producing cell. We have chemically synthesized the full precursor of carnobacteriocin B2, precarnobacteriocin (preCbnB2), which has a C-terminal amide rather than a carboxyl, and also produced preCbnB2(1-64), which is missing two amino acid residues at the C-terminus (Arg65 and Pro66), via expression in Escherichia coli as a maltose-binding protein fusion that is then cut with Factor Xa. PreCbnB2(1-64) is readily labeled with (15)N and (13)C for NMR studies using the latter approach. Multidimensional NMR analysis of preCbnB2(1-64) shows that, like the parent bacteriocin, it exists as a random coil in water but assumes a defined conformation in water/trifluoroethanol mixtures. In 70 : 30 trifluoroethanol/water, the 3D structure of the preCbnB2 section corresponding to the mature bacteriocin is essentially the same as reported previously by us for carnobacteriocin B2 (CbnB2). This structure maintains the highly conserved alpha-helix corresponding to residues 20-38 of CbnB2 that is believed to be responsible for interaction with a target receptor in sensitive cells, including Listeria monocytogenes. PreCbnB2 also has a second alpha-helix from residues 3-13 (i.e. -15 to -5 relative to CbnB2) in the leader section of the peptide. This helix appears to be conserved in related type IIa bacteriocin precursors based on sequence analysis. It is likely to be a key recognition element for export and processing, and is probably responsible for the considerably reduced antimicrobial activity of preCbnB2. The latter effect may assist the producing cell in avoiding the toxic effects of the bacteriocin. This is the first 3D structure determined for a prebacteriocin from lactic acid bacteria.