Functional Interactions between Coexisting Toxin-Antitoxin Systems of the ccd Family in Escherichia coli O157:H7

Abstract

Toxin-antitoxin (TA) systems are widely represented on mobile genetic elements as well as in bacterial chromosomes. TA systems encode a toxin and an antitoxin neutralizing it. We have characterized a homolog of the ccd TA system of the F plasmid ( ccd _F ) located in the chromosomal backbone of the pathogenic O157:H7 Escherichia coli strain ( ccd _O157 ). The ccd _F and the ccd _O157 systems coexist in O157:H7 isolates, as these pathogenic strains contain an F-related virulence plasmid carrying the ccd _F system. We have shown that the chromosomal ccd _O157 system encodes functional toxin and antitoxin proteins that share properties with their plasmidic homologs: the CcdB _O157 toxin targets the DNA gyrase, and the CcdA _O157 antitoxin is degraded by the Lon protease. The ccd _O157 chromosomal system is expressed in its natural context, although promoter activity analyses revealed that its expression is weaker than that of ccd _F . ccd _O157 is unable to mediate postsegregational killing when cloned in an unstable plasmid, supporting the idea that chromosomal TA systems play a role(s) other than stabilization in bacterial physiology. Our cross-interaction experiments revealed that the chromosomal toxin is neutralized by the plasmidic antitoxin while the plasmidic toxin is not neutralized by the chromosomal antitoxin, whether expressed ectopically or from its natural context. Moreover, the ccd _F system is able to mediate postsegregational killing in an E. coli strain harboring the ccd _O157 system in its chromosome. This shows that the plasmidic ccd _F system is functional in the presence of its chromosomal counterpart.