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Injectisome T3SS subunits as potential chaperones in the extracellular export of Pectobacterium carotovorum subsp. carotovorum bacteriocins Carocin S1 and Carocin S3 secreted via flagellar T3SS.

ABSTRACT: Pectobacterium carotovorum subsp. carotovorum (Pcc) causes soft-rot disease in a wide variety of plants resulting in economic losses worldwide. It produces various types of bacteriocin to compete against related plant pathogens. Studies on how bacteriocins are extracellularly secreted are conducted to understand the mechanism of interbacterial competition. In this study, the secretion of the low-molecular-weight bacteriocins (LMWB) Carocin S1 and Carocin S3 produced by a multiple-bacteriocin producing strain of Pcc, 89-H-4, was investigated. Tn5 insertional mutagenesis was used to generate a mutant, TH22-6, incapable of LMWBs secretion. Sequence and homology analyses of the gene disrupted by transposon Tn5 insertion revealed that the gene sctT, an essential component of the injectisome type III secretion machinery (T3aSS), is required for the secretion of the bacteriocins. This result raised a question regarding the nature of the secretion mechanism of Pcc bacteriocins which was previously discovered to be secreted via T3bSS, a system that utilizes the bacterial flagellum for extracellular secretions. Our previous report has shown that bacteriocin Carocin S1 cannot be secreted by mutants that are defective of T3bSS-related genes such as flhA, flhC, flhD and fliC. We knocked out several genes making up the significant structural components of both T3aSS and T3bSS. The findings led us to hypothesize the potential roles of the T3aSS-related proteins, SctT, SctU and SctV, as flagellar T3SS chaperones in the secretion of Pcc bacteriocins. This current discovery and the findings of our previous study helped us to conceptualize a unique Type III secretion system for bacteriocin extracellular export which is a hybrid of the injectisome and flagellar secretion systems.

SUBMITTER: Wu HP

PROVIDER: S-EPMC8672553 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:BackgroundPectobacterium carotovorum subsp. carotovorum belongs to the Enterobacteriaceae family, which causes soft-rot disease in numerous plants worldwide resulting in significant economic losses. Results from our previous studies showed that the strain H-rif-8-6 produces low-molecular-weight bacteriocin (LMWB) Carocin S1. Interestingly, TH22-10, the caroS1K:Tn5 insertional mutant in H-rif-8-6, loses Carocin S1 producing ability, but still produces other LMWBs which the indicator strain SP33 can detect. The SP33 is one of the many strains that are sensitive toward the cytotoxic effects of Carocin S3K, but not Carocin S1. The result revealed that H-rif-8-6 is a multiple-bacteriocin producing strain.ResultsIn this study, a 4.1-kb DNA fragment was isolated from the chromosomal DNA of Pcc strain, H-rif-8-6, by a DNA probe using the caroS1K gene as the template. DNA sequencing and analysis by GenBank revealed two complete open reading frames (ORFs), designated ORF1 and ORF2, which were identified within the sequence fragment. ORF1 and ORF2, similar to the identified carocin S2 genes, encode the killer (Carocin S3K) and the immunity (Carocin S3I) proteins, respectively, which were homologous to the colicin E3 gene. Carocin S3K and Carocin S3I were expressed, isolated, and purified in Escherichia coli BL21 after subcloning of the expression plasmid pGS3KI or pGSK3I. SDS-PAGE analysis showed that the relative masses of Carocin S3K and Carocin S3I were 95.6 kDa and 10.2 kDa, respectively. The results reveal that Carocin S3K has higher antimicrobial and specific antimicrobial activities for Pcc along with a nuclease activity than Carocin S3I. However, Carocin S3I inhibits the activity of Carocin S3K. Interestingly, a high concentration of Carocin S3I protein is also a DNA nuclease, and Carocin S3K also inhibits its activity.ConclusionThis study showed that another type of bacteriocin was found in Pectobacterium carotovorum. This new type of bacteriocin, Carocin S3, has the killer protein, Carocin S3K, and the immunity protein, Carocin S3I.

Project description:Pectobacterium carotovorum subsp. carotovorum is a necrotrophic plant pathogen that secretes plant cell wall-degrading enzymes (PCWDEs) that cause soft rot disease in various crops. Bacteriophages have been under consideration as harmless antibacterial agents to replace antibiotics and copper-based pesticides. However, the emergence of bacteriophage resistance is one of the main concerns that should be resolved for practical phage applications. In this study, we developed a phage cocktail with three lytic phages that recognize colanic acid (phage POP12) or flagella (phages POP15 and POP17) as phage receptors to minimize phage resistance. The phage cocktail effectively suppressed the emergence of phage-resistant P. carotovorum subsp. carotovorum compared with single phages in in vitro challenge assays. The application of the phage cocktail to napa cabbage (Brassica rapa subsp. pekinensis) resulted in significant growth retardation of P. carotovorum subsp. carotovorum (P < 0.05) and prevented the symptoms of soft rot disease. Furthermore, phage cocktail treatments of young napa cabbage leaves in a greenhouse environment indicated effective prevention of soft rot disease compared to that in the nonphage negative control. We isolated 15 phage-resistant mutants after a phage cocktail treatment to assess the virulence-associated phenotypes compared to those of wild-type (WT) strain Pcc27. All mutants showed reduced production of four different PCWDEs, leading to lower levels of tissue softening. Ten of the 15 phage-resistant mutants additionally exhibited decreased swimming motility. Taken together, these results show that the phage cocktail developed here, which targets two different types of phage receptors, provides an effective strategy for controlling P. carotovorum subsp. carotovorum in agricultural products, with a potential ability to attenuate P. carotovorum subsp. carotovorum virulence. IMPORTANCE Pectobacterium carotovorum subsp. carotovorum is a phytopathogen that causes soft rot disease in various crops by producing plant cell wall-degrading enzymes (PCWDEs). Although antibiotics and copper-based pesticides have been extensively applied to inhibit P. carotovorum subsp. carotovorum, the emergence of antibiotic-resistant bacteria and demand for harmless antimicrobial products have emphasized the necessity of finding alternative therapeutic strategies. To address this problem, we developed a phage cocktail consisting of three P. carotovorum subsp. carotovorum-specific phages that recognize colanic acids and flagella of P. carotovorum subsp. carotovorum. The phage cocktail treatments significantly decreased P. carotovorum subsp. carotovorum populations, as well as soft rot symptoms in napa cabbage. Simultaneously, they resulted in virulence attenuation in phage-resistant P. carotovorum subsp. carotovorum, which was represented by decreased PCWDE production and decreased flagellum-mediated swimming motility. These results suggested that preparations of phage cocktails targeting multiple receptors would be an effective approach to biocontrol of P. carotovorum subsp. carotovorum in crops.

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Injectisome T3SS subunits as potential chaperones in the extracellular export of Pectobacterium carotovorum subsp. carotovorum bacteriocins Carocin S1 and Carocin S3 secreted via flagellar T3SS.

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