Abstract
The Xylella fastidiosa subsp pauca strain 9a5c is a Gram-negative, xylem-limited bacterium that is able to form a biofilm and affects citrus crops in Brazil. Some genes are considered to be involved in biofilm formation, but the specific mechanisms involved in this process remain unknown. This limited understanding of how some bacteria form biofilms is a major barrier to our comprehension of the progression of diseases caused by biofilm-producing bacteria. Several investigations have shown that the toxin-antitoxin (TA) operon is related to biofilm formation. This operon is composed of a toxin with RNAse activity and its cognate antitoxin. Previous reports have indicated that the antitoxin is able to inhibit toxin activity and modulate the expression of the operon as well as other target genes involved in oxidative stress and mobility. In this study, we characterize a toxin-antitoxin system consisting of XfMqsR and XfYgiT, respectively, from X. fastidiosa subsp. pauca strain 9a5c. These proteins display a high similarity to their homologs in X. fastidiosa strain Temecula and a predicted tridimensional structure that is similar to MqsR-YgiT from Escherichia coli. The characterization was performed using in vitro assays such as analytical ultracentrifugation (AUC), size exclusion chromatography, isothermal titration calorimetry, and Western blotting. Using a fluorometric assay to detect RNAses, we demonstrated that XfMqsR is thermostable and can degrade RNA. XfMqsR is inhibited by XfYgiT, which interacts with its own promoter. XfYgiT is known to be localized in the intracellular compartment; however, we provide strong evidence that X. fastidiosa secretes wild-type XfYgiT into the extracellular environment via outer membrane vesicles, as confirmed by Western blotting and specific immunofluorescence labeling visualized by fluorescence microscopy. Taken together, our results characterize the TA system from X. fastidiosa strain 9a5c, and we also discuss the possible influence of wild-type XfYgiT in the cell.
Highlights
Xylella fastidiosa subsp pauca strain 9a5c is a Gram-negative bacteria and the causal agent of citrus variegated chlorosis (CVC)
TA systems are widely distributed in bacteria and archaea (Park et al, 2013) and are considered to have been acquired by horizontal gene transfer because the genes are embedded in a W-V prophage region
Bacterial pathogenicity may be directly proportional to the number of TA systems in the genome compared with the genomes of species that are not involved in epidemics (Georgiades and Raoult, 2011)
Summary
Xylella fastidiosa subsp pauca strain 9a5c is a Gram-negative bacteria and the causal agent of citrus variegated chlorosis (CVC). Biofilm formation results in water deficiency, limitations in nutrient transport and death during later stages of infection (Rodrigues et al, 2013) This structure is involved in the pathogenicity of several species such as X. fastidiosa (Caserta et al, 2010; Voegel et al, 2010; Janissen et al, 2015), Neisseria meningitides (Arenas et al, 2015), Streptococcus pneumonia (Domenech et al, 2015), Salmonella enteric (O’Leary et al, 2015), and Pseudomonas syringae (Chowdhury and Jagannadham, 2013), conferring resistance to antibiotics and other chemicals used to control bacterial populations
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