Abstract
Toxin–antitoxin systems are commonly found on plasmids and chromosomes of bacteria and archaea. These systems appear as biscystronic genes encoding a stable toxin and a labile antitoxin, which protects the cells from the toxin’s activity. Under specific, mostly stressful conditions, the unstable antitoxin is degraded, the toxin becomes active and growth is arrested. Using genome analysis we identified a putative toxin–antitoxin encoding system in the genome of the plant pathogen Acidovorax citrulli. The system is homologous to vapB–vapC systems from other bacterial species. PCR and phylogenetic analyses suggested that this locus is unique to group II strains of A. citrulli. Using biochemical and molecular analyses we show that A. citrulli VapBC module is a bona-fide toxin–antitoxin module in which VapC is a toxin with ribonuclease activity that can be counteracted by its cognate VapB antitoxin. We further show that transcription of the A. citrulli vapBC locus is induced by amino acid starvation, chloramphenicol and during plant infection. Due to the possible role of TA systems in both virulence and dormancy of human pathogenic bacteria, studies of these systems are gaining a lot of attention. Conversely, studies characterizing toxin–antitoxin systems in plant pathogenic bacteria are lacking. The study presented here validates the activity of VapB and VapC proteins in A. citrulli and suggests their involvement in stress response and host–pathogen interactions.
Highlights
The Gram-negative bacterium Acidovorax citrulli is a seed-borne pathogen responsible for bacterial fruit blotch (BFB), a threatening disease of cucurbits worldwide (Schaad et al, 2003)
Analysis of the annotated genome of the group II strain of A. citrulli AAC00-1 revealed the presence of a putative vapBC operon in the chromosome of this bacterium (Aave_0579 and Aave_0580; Figure 1A)
Additional control samples consisted of the buffers that were used in the reaction without VapC, these were: refolding buffer used in the dialysis, reaction buffer 50 mM Tris-HCl pH 7 and 6 mM MgCl
Summary
The Gram-negative bacterium Acidovorax citrulli is a seed-borne pathogen responsible for bacterial fruit blotch (BFB), a threatening disease of cucurbits worldwide (Schaad et al, 2003). Under favorable conditions, this bacterium spreads rapidly throughout nurseries and in the field leading to seedling blight or, at a later stage, fruit rot. Chemical control of the disease in the field has only limited efficiency and to date, there are no sources of BFB resistance (Bahar et al, 2009b; Burdman and Walcott, 2012). Understanding the mechanisms that promote plant tissue colonization, virulence and spread of A. citrulli is important for developing efficient tools to manage BFB.
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