Type II toxin/antitoxin system ParESO/CopASO stabilizes prophage CP4So in <italic>Shewanella oneidensis</italic>.

Summary: 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, ParE_SO/CopA_SO, in the CP4So prophage in <italic>Shewanella oneidensis</italic>. We demonstrated that ParE_SO/CopA_SO plays a critical role in the maintenance of CP4So in host cells after its excision. The toxin ParE_SO inhibited cell growth, resulting in filamentous growth and eventually cell death. The antitoxin CopA_SO neutralized the toxicity of ParE_SO 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′‐ GTAN TAC (N)₃ GTAN TAC‐3′]. CopA_SO also repressed transcription of another TA system PemK_SO<italic>/</italic>PemI_SO in megaplasmid pMR‐1 of <italic>S. oneidensis</italic> through binding to a highly similar DNA motif in its promoter region. CopA_SO homologs are widely spread in <italic>Shewanella</italic> and other <italic>Proteobacteria</italic>, 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. [ABSTRACT FROM AUTHOR]