Systematic transcriptome analysis allows the identification of new type I and type II Toxin/Antitoxin systems located in the superintegron of Vibrio cholerae

Abstract

Vibrio cholerae N16961 genome encodes 18 type II Toxin/Antitoxin (TA) systems, all but one located inside gene cassettes of its chromosomal superintegron (SI). This study aims to investigate additional TA systems in this genome. We screened for all two-genes operons of uncharacterized function by analyzing previous RNAseq data. Assays on nine candidates, revealed one additional functional type II TA encoded by the VCA0497-0498 operon, carried inside a SI cassette. We showed that VCA0498 antitoxin alone and in complex with VCA0497 represses its own operon promoter. VCA0497-0498 is the second element of the recently identified dhiT/dhiA superfamily uncharacterized type II TA system. RNAseq analysis revealed that another SI cassette encodes a novel type I TA system: VCA0495 gene and its two associated antisense non-coding RNAs, ncRNA495 and ncRNA496. Silencing of both antisense ncRNAs lead to cell death, demonstrating the type I TA function. Both VCA0497 and VCA0495 toxins do not show any homology to functionally characterized toxins, however our preliminary data suggest that their activity may end up in mRNA degradation, directly or indirectly. Our findings increase the TA systems number carried in this SI to 19, preferentially located in its distal end, confirming their importance in this large cassette array.