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.Toxin-antitoxin (TA) proteic systems were originally discovered on low-copy-number plasmids (for reviews on TA systems, see references 11, 22, 24, and 29). They are composed of two genes organized in an operon encoding a toxin and an antitoxin that antagonizes it. The expression of the TA genes is autoregulated at the transcriptional level; the antitoxin acts as a repressor and the toxin often as a corepressor. The antitoxin is an unstable protein degraded by an ATP-dependent protease, while the toxin is a stable protein that inhibits an essential cellular process (e.g., replication and translation). TA systems contribute to plasmid stability by a mechanism called postsegregational killing (PSK). PSK relies on the differential stabilities of the antitoxin and toxin proteins and leads to the killing of daughter bacteria that did not receive a plasmid copy at cell division (31, 50, 53).Recent computational analyses have shown that TA systems are widely represented in eubacterial and archaebacterial chromosomes, suggesting a role for horizontal gene transfer in the spread of these genes (5, 6, 38). The localization of chromosomal TA systems is quite varied. Some are localized within exogenous DNA islands like phages (relBE K-12 in the cryptic lambdoid Qin prophage of Escherichia coli MG1655) (40), transposons (relBE homolog in Tn5401 of Bacillus thuringiensis (23), and superintegrons (relBE, parDE, phd-doc, and hig...
The origin and the evolution of toxin-antitoxin (TA) systems remain to be uncovered. TA systems are abundant in bacterial chromosomes and are thought to be part of the flexible genome that originates from horizontal gene transfer. To gain insight into TA system evolution, we analyzed the distribution of the chromosomally encoded ccd O157 system in 395 natural isolates of Escherichia coli. It was discovered in the E. coli O157:H7 strain in which it constitutes a genomic islet between two core genes ( folA and apaH). Our study revealed that the folA-apaH intergenic region is plastic and subject to insertion of foreign DNA. It could be composed (i) of a repetitive extragenic palindromic (REP) sequence, (ii) of the ccd O157 system or subtle variants of it, (iii) of a large DNA piece that contained a ccdA O157 antitoxin remnant in association with ORFs of unknown function, or (iv) of a variant of it containing an insertion sequence in the ccdA O157 remnant. Sequence analysis and functional tests of the ccd O157 variants revealed that 69% of the variants were composed of an active toxin and antitoxin, 29% were composed of an active antitoxin and an inactive toxin, and in 2% of the cases both ORFs were inactive. Molecular evolution analysis showed that ccdB O157 is under neutral evolution, suggesting that this system is devoid of any biological role in the E. coli species.
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