Characterization of a New Bacteriocin, Carocin D, from Pectobacterium carotovorum subsp. carotovorum Pcc21

Abstract

ABSTRACT Two different bacteriocins, carotovoricin and carocin S1, had been found in Pectobacterium carotovorum subsp. carotovorum , which causes soft-rot disease in diverse plants. Previously, we reported that the particular strain Pcc21, producing only one high-molecular-weight bacteriocin, carried a new antibacterial activity against the indicator strain Pcc3. Here, we report that this new antibacterial activity is due to a new bacteriocin produced by strain Pcc21 and named carocin D. Carocin D is encoded by the caro DK gene located in the genomic DNA together with the caro DI gene, which seems to encode an immunity protein. N-terminal amino acid sequences of purified carocin D were determined by Edman degradation. In comparison with the primary translation product of caro DK, it was found that 8 amino acids are missing at the N terminus. This finding proved that carocin D is synthesized as a precursor peptide and that 8 amino acids are removed from its N terminus during maturation. Carocin D has two putative translocation domains; the N-terminal and C-terminal domains are homologous to those of Escherichia coli colicin E3 and Pseudomonas aeruginosa S-type pyocin, respectively. When caro DK and caro DI genes were transformed into carocin D-sensitive bacteria such as Pcc3, the bacteria became resistant to this bacteriocin. Carocin D has one putative DNase domain at the extreme C terminus and showed DNase activity in vitro . This bacteriocin had slight tolerance to heat but not to proteases. The caro DK gene was present in only 5 of 54 strains of P. carotovorum subsp. carotovorum . These results indicate that carocin D is a third bacteriocin found in P. carotovorum subsp. carotovorum , and this bacteriocin can be readily expressed in carocin D-sensitive nonpathogenic bacteria, which may have high potential as a biological control agent in the field.