Abstract
Plant cell wall degrading enzymes (PCWDEs) are the primary virulence determinants of soft rotting bacteria such as the potato pathogen, Pectobacterium atrosepticum. The regulation of secondary metabolite (Rsm) system controls production of PCWDEs in response to changing nutrient conditions. This work identified a new suppressor of an rsmB mutation – ECA1172 or rsmS (rsmBsuppressor). Mutants defective in rsmB (encoding a small regulatory RNA), show reduced elaboration of the quorum sensing molecule (N-3-oxohexanoyl-homoserine lactone; OHHL) and PCWDEs. However, OHHL and PCWDE production were partially restored in an rsmB, rsmS double mutant. Single rsmS mutants, overproduced PCWDEs and OHHL relative to wild type P. atrosepticum and exhibited hypervirulence in potato. RsmS overproduction also resulted in increased PCWDEs and OHHL. Homology searches revealed rsmS conservation across pathogens such as Escherichia coli (ybaM), Dickeya solani, Klebsiella pneumoniae and Shigella flexneri. An rsmS mutant of Pectobacterium carotovorum ATCC39048 showed bypass of rsmB-dependent repression of PCWDEs and OHHL production. P. carotovorum ATCC39048 produces the β-lactam antibiotic, 1-carbapen-2-em-3-carboxylic acid (a carbapenem). Production of the antibiotic was repressed in an rsmB mutant but partially restored in an rsmB, rsmS double mutant. This work highlights the importance of RsmS, as a conserved pleiotropic regulator of virulence and antibiotic biosynthesis.
Highlights
Members of the Pectobacteriaceae are the causative agents of soft rot in potato tubers and blackleg in potato plants
Swimming motility is an important virulence determinant in Pectobacterium atrosepticum SCRI1043 (Pba)[2,10,24], we examined whether mutation of rsmS affected flagellum-dependent swimming
We found no significant difference in swimming motility between wild type Pba, an rsmB mutant, an rsmS mutant or an rsmB rsmS double mutant
Summary
Members of the Pectobacteriaceae are the causative agents of soft rot in potato tubers and blackleg in potato plants These Gram-negative bacteria, such as Pectobacterium atrosepticum SCRI1043 (Pba) and Pectobacterium carotovorum (Pcc), are responsible for diminished yields due to diseased plants, seed potato infection, and soft rot decay of tubers post-harvest during storage. Both soft rot and blackleg diseases are caused by the secretion of plant cell wall degrading enzymes (PCWDEs), such as pectate lyases, polygalacturonase, cellulases and proteases[1]. In the absence of OHHL, VirR binds the promoter of rsmA, another important virulence regulator, activating its transcription and linking the Rsm and QS systems[9]. We postulated that there might be other suppressors of rsmB, still to be identified
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