Abstract
Mutational, nucleotide sequence, and transcriptional analyses of a 10-kb fragment (carnobacteriocin locus) from the 61-kb plasmid of Carnobacterium piscicola LV17B demonstrated the presence of two gene clusters (cbnXY and cbnSKRTD) upstream of the previously sequenced carnobacteriocin B2 structural and immunity genes (cbnB2 and cbiB2). Deduced products of cbnK and cbnR have sequence similarity to proteins of Agr-type two-component signal transduction systems, and those of cbnT and cbnD have sequence similarity to proteins of signal sequence-independent secretion systems. Deduced products of cbnX, cbnY, and cbnS are class II-type bacteriocin precursors with potential leader peptides containing double-glycine cleavage sites. Genetic analysis indicated that the 10-kb locus contains information required for the production of, and immunity to, the plasmid-encoded carnobacteriocin B2 and the chromosomally encoded carnobacteriocin BM1. In addition, this locus is involved in the production of at least one additional antimicrobial compound and an inducer factor that plays a role in the regulation of carnobacteriocin B2. Transcription analysis indicated that the operons cbnXY, cbnB2-cbiB2, and cbnBM1-cbiBM1 (with the latter encoding carnobacteriocin BM1 and its immunity protein on the chromosome) and two small transcripts containing cbnS are transcribed only in induced cultures. These transcripts are coregulated and subject to inducer-mediated transcriptional control. Similar regulation of the cbn operons is mirrored by the similarity in the nucleotide sequence of their promoter regions, all of which contain two imperfect direct repeats resembling those in Agr-like regulated promoters upstream of the transcription start sites.