Type II toxin/antitoxin system ParESO/CopASO stabilizes prophage CP4So in Shewanella oneidensis

Toxin/antitoxin (TA) loci are commonly found in mobile genetic elements such as plasmids and prophages. However, the physiological functions of these TA loci in prophages and cross-regulation among these TA loci remain largely unexplored. Here, we characterized a newly discovered type II TA pair, ParESO /CopASO , in the CP4So prophage in Shewanella oneidensis. We demonstrated that ParESO /CopASO plays a critical role in the maintenance of CP4So in host cells after its excision. The toxin ParESO inhibited cell growth, resulting in filamentous growth and eventually cell death. The antitoxin CopASO neutralized the toxicity of ParESO through direct protein-protein interactions and repressed transcription of the TA operon by binding to a DNA motif in the promoter region containing two inverted repeats [5'-GTANTAC (N)3 GTANTAC-3']. CopASO also repressed transcription of another TA system PemKSO /PemISO in megaplasmid pMR-1 of S. oneidensis through binding to a highly similar DNA motif in its promoter region. CopASO homologs are widely spread in Shewanella and other Proteobacteria, either as a component of a TA pair or as orphan antitoxins. Our study thus illustrated the cross-regulation of the TA systems in different mobile genetic elements and expanded our understanding of the physiological function of TA systems.