Study of the functional role of the toxin-antitoxin system XF2490/XF2491 of Xylella fastidiosa under copper stress

Through the formation of persister cells, bacteria are capable of exhibiting tolerance to multidrug and other environmental stresses without undergoing genetic changes. The toxin-antitoxin (TA) systems are involved in the process of persister cells formation, since they are able to induce cell dormancy. Among the TA systems, the mqsRA system has been described in Escherichia coli as related both to the formation of persister cells, as with biofilm formation. In X. fastidiosa, the causal agent of Citrus Variegated Chlorosis (CVC), the mqsRA TA system has also been previously observed to be highly induced in persister cells of this organism activated by copper stress. Thus, in this study we evaluated the functional role of this TA system in X. fastidiosa. By overexpressing the mqsR toxin in X. fastidiosa, it was seen that this system is also related to the lifestyle of this bacterium, as it induced biofilm formation and reduced twitching motility. These results corroborated with the gene expression profile, since the mqsR overexpression induced genes related to growth in biofilm and repressed genes related to cell movement, indicating the regulatory ability of this TA system. Thus, these observed characteristics resulted in a reduced pathogenicity of this bacterium in planta. The formation of persister cells under copper stress also increased with the overexpression of mqsR, which was seen by increased cell survival and increased formation of elongated cells. Through the analysis of the expression of mqsR and mqsA by real-time quantitative PCR and of the proteins by Western blot, we observed that this system probably possess a self-regulatory mechanism that allows expressing the toxin and antitoxin in the most beneficial ratio for the cell to oppose the stress and resume growth quickly when conditions become favorable. Thus, the results suggest that this TA system has a key role in the adaptation and survival of X. fastidiosa in its different hosts (AU)