Bacteriocin 51 (Bac 51) is encoded on the mobile plasmid pHY (6,037 bp), which was isolated from vancomycin-resistant Enterococcus faecium VRE38. Bacteriocin 51 is active against E. faecium, E. hirae, and E. durans. Sequence analysis of pHY showed that it encodes nine open reading frames (ORFs) from ORF1 to ORF9 (in that order). Genetic analysis suggested that ORF1 and ORF2, which were designated bacA and bacB, respectively, are the bacteriocin and immunity genes. bacA encodes a 144-amino-acid protein. The deduced BacA protein has a typical signal sequence at its amino terminus, and a potential signal peptidase-processing site corresponding to the V-E-A sequence is located between the 37th and 39th amino acids. The predicted mature BacA protein consists of 105 amino acids. A potential promoter sequence was identified upstream of the start codon. bacB encodes a 55-amino-acid protein. No obvious promoter or terminator sequence was identified between bacA and bacB. Northern blot analysis of bacA and bacB with a bacA RNA probe produced a transcript of approximately 700 nucleotides, which corresponded to the combined nucleotide sizes of bacA and bacB, indicating that transcription was initiated from the promoter upstream of bacA, continued through bacB, and was terminated at the terminator downstream of bacB. The transcription start site was determined to be the T nucleotide located 6 nucleotides downstream from the -10 promoter sequence. These results indicate that bacA and bacB constitute an operon and that bacA is the bacteriocin structural gene while bacB is the immunity gene. The purified C-terminally His tagged BacA protein of Bac 51 showed bacteriostatic activity against the indicator strain. The purified C-terminally His tagged BacA protein of Bac 32 (whose mature BacA protein has 54 amino acids) and the culture filtrates of the Bac 31- and Bac 43-producing E. faecalis strain FA2-2 showed bactericidal activity. Bac 31 and Bac 43 are pore-forming bacteriocins, unlike the newly characterized bacteriocin Bac 51.