Comparative genomics reveals the presence of putative toxin–antitoxin system in Wolbachia genomes

Multiple toxin–antitoxin (TA) systems are housed in different locations within the bacterial genome and are known to be associated with various cellular processes and stress-related adaptation. In endosymbionts, although, the TA system has scarce occurrence but studies have highlighted its presence in enhancing host–symbiont interactions. Wolbachia, an obligate endosymbiont, has recently been proposed as a biocontrol agent which may be helpful in controlling vector-borne diseases. There are reports suggesting the role of TA system in inducing cytoplasmic incompatibility in case of Wolbachia, however, the underlying mechanism is still not known. The present study, therefore, aims at exploring the diversity of TA system in four novel (sourced from India) and three reference genomes (NCBI) of Wolbachia strains. Interestingly, we found several putative toxins and antitoxins of RelEB family of Type II TA system in these Wolbachia genomes. The results show wMel genome possessed more number of putative TA loci than wRi genome. In addition, searching through the other sequenced Wolbachia genomes in NCBI, a complete absence of TA system was observed in Wolbachia-infected nematodes. The sequence-wide analysis of all the putative RelEB proteins present amongst the Wolbachia endosymbiont and within the free-living bacterial genomes reveal strain-specific similarities and conserved sequences. However, large amount of sequence diversity was observed between Wolbachia and free-living bacteria. Understanding this sequence variation may help shed light on the differences between these two forms of bacteria and could also explain their niche preferences.