Extracellular secretion of Carocin S1 in Pectobacterium carotovorum subsp. carotovorum occurs via the type III secretion system integral to the bacterial flagellum

A mutant (flhC::Tn5) that did not secrete the low-molecular-weight bacteriocin (LMWB), Carocin S1, was generated by Tn5 insertional mutagenesis. Sequence analysis indicated that this insertion disrupted open reading frame 2 (ORF2) and ORF3 of this strain. Deletion and rescue experiments indicated that ORF2 and ORF3 were both required for extracellular LMWB secretion. The ORF2 and ORF3 sequences showed high homology with the flhD and flhC gene sequences of Pectobacterium carotovorum subsp. atroseptica, Serratia marcescens, Yersinia enterocolitica, and Escherichia coli, indicating that they likely encoded key regulatory components of the type III flagella secretion system.Thus, the extracellular export of Carocin S1 by Pectobacterium carotovorum subsp. carotovorum appears to utilize the type III secretion system integral to bacterial flagella.Pectobacterium carotovorum subsp. carotovorum is a phytopathogenic enterobacterium responsible for soft rot, a disease characterized by extensive plant tissue maceration caused by a variety of secreted enzymes. The major pathogenicity determinants are an arsenal of extracellular pectinases, including several pectate lyase isozymes: pectin lyase, pectin methylesterase, and pectin polygalacturonase. In addition, a range of other degradative enzymes, such as cellulase and proteases, play equivocal roles in virulence [1]. Pectobacterium carotovorum subsp. carotovorum also produces one or more antibacterial substances called bacteriocins, which enhance their competitiveness with other related rival species [2]. The ability of this bacterial species to produce bacteriocin has been exploited in many biological control programs for the soft-rot disease of Chinese cabbage [3-5]. In view of this, identification and cloning of the gene(s) controlling bacteriocin production may facilitate the development of wider and more innovative control methods, such as the cloning of these gene(s) into Chinese cabbage, tobacco, and other susceptible pla