THE ESCHERICHIA-COLI RELBE GENES BELONG TO A NEW TOXIN-ANTITOXIN GENEFAMILY

Toxin-antitoxin systems are defined as a group of plasmid- and chromos ome-encoded loci that specify a cell toxin and a protein antitoxin. Pl asmid-encoded toxin-antitoxin systems stabilize their replicons by kil ling plasmid-free cells. Here, we show that the relBE genes of Escheri chia coli K-12 have all the basic features previously connected with t oxin-antitoxin systems: (i) relE encodes a cytotoxin lethal or inhibit ory to host cells; (ii) relB encodes an antitoxin that prevents the le thal action of the relE-encoded toxin; (iii) the relBE genes stabilize a mini-R1 test plasmid; and (iv) the RelB antitoxin autoregulates the reIBEF operon at the level of transcription. Using database searching , we found relBE homologues on the chromosomes of E. coli K-12, Haemop hilus influenzae and Vibrio cholerae. A fifth relBE homologue was iden tified on the enterotoxin encoding E. coli plasmid P307. Indirect evid ence suggests that the toxicity of RelE may be related to the inhibiti on of protein synthesis. Based on these observations, we propose a mod el that explains the delayed relaxed phenotype associated with mutatio ns in relB.