Functional interactions between coexisting toxin-antitoxin systems of the ccd family in Escherichia coli O157:H7

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.sub.F]) located in the chromosomal backbone of the pathogenic O157:H7 Escherichia coli strain ([ccd.sub.O157]). The [ccd.sub.F] and the [ccd.sub.O157] systems coexist in O157:H7 isolates, as these pathogenic strains contain an F-related virulence plasmid carrying the [ccd.sub.F] system. We have shown that the chromosomal [ccd.sub.O157] system encodes functional toxin and antitoxin proteins that share properties with their plasmidic homologs: the CcdB[O.sub.157] toxin targets the DNA gyrase, and the CcdA[O.sub.157] antitoxin is degraded by the Lon protease. The [ccd.sub.O157] chromosomal system is expressed in its natural context, although promoter activity analyses revealed that its expression is weaker than that of [ccd.sub.F] [ccd.sub.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 ccdr system is able to mediate postsegregational killing in an E. coli strain harboring the [ccd.sub.O157] system in its chromosome. This shows that the plasmidic [ccd.sub.F] system is functional in the presence of its chromosomal counterpart.