R n 1Department of Biochemistry 2Centre for Bacterial Cell

RelB, the ribbon–helix–helix (RHH) repressor encoded by the relBE toxin– doi:10.1016/j.jmb.2009.09.006 Available online at wwwIntroduction Toxin–antitoxin (TA) loci are abundant gene cassettes present, often in multiple copies, on plasmids and chromosomes of Bacteria and Archaea.1 TA loci encode a stable “toxin ” that inhibits translation or replication and an unstable antitoxin that neutralizes the toxin. Ectopic produc-tion of the toxins is highly inhibitory to cell growth, but the antitoxins form complexes with the toxins and thereby counterbalance toxin activity. Activa-tion of TA loci depends on cellular proteases such as Lon or Clp that degrade the antitoxins.2,3 Two major classes of toxins have been identified based on their cellular targets. One class of toxins inhibits translation by mRNA cleavage (i.e., RelEreceived in revised for