The characteristic features of TA loci are that they comprise a <

The characteristic features of TA loci are that they comprise a VX-809 mouse TA gene pair in a bicistronic operon, consisting of an upstream antitoxin and a downstream toxin gene. Normally, two small proteins, a stable toxin and a labile antitoxin, associate tightly so as to keep the toxin component inert (Kwong et al., 2010). The putative role of the antitoxin gene product has been widely discussed, suggesting that there are at least two types of antitoxin. In Type I systems, the antitoxin is an RNA molecule

that neutralized the toxin translation and in Type II systems the antitoxin is a small labile protein that binds avidly to the toxin, inhibiting its activity or by downregulating its expression (Hayes, 2003). On the other hand, the toxins of Type I systems are small, hydrophobic proteins that confer their toxicity by damaging cell membranes, while Type II toxins damage particularly either DNA or RNA molecules (Van Melderen & Saavedra de Bast, 2009). In short, whatever their real function is, TA modules

can attack cells from within (Engelberg-Kulka et al., 2005) and a number of different intracellular targets have already been identified (Hayes, 2003). In recent years, the TA system has been consistently associated with a crucial regulatory process in living organisms better known as PCD. PCD is an active process that results in cell suicide and is an essential mechanism in multicellular organisms, required for Tyrosine Kinase Inhibitor Library manufacturer the elimination of superfluous or potentially harmful cells (Engelberg-Kulka & Glaser, 1999). PCD is currently used to refer

to any form of cell death mediated by an intracellular program, no matter what triggers it and whether or not it displays the characteristics of apoptosis (Hengartner & Bryant, 2000). The recent discovery of TA modules in many bacteria suggests that PCD may be a general phenomenon in bacteria (Picardeau et al., 2001). In this study, we report the presence of a TA locus in the genome of Piscirickettsia salmonis, a Gram-negative fish bacterial pathogen that has affected the salmonid industry since 1989 (Bravo & Campos, 1989). Piscirickettsia salmonis, the aetiological agent of the Salmonid Rickettsial Septicaemia (SRS) or Piscirickettsiosis, belongs to the Gammaproteobacteria group (Fryer & Hedrick, 2003) and was recently reclassified as a facultative intracellular organism (Mauel et al., 2008; Mikalsen et al., 2008; Gómez et al., 2009). Piscirickettsiosis was first reported in coho salmon (Oncorhynchus kisutch) (Bravo & Campos, 1989), but infectivity has also been demonstrated in cultured salmonid species such as the Atlantic salmon (Salmo salar), Chinook salmon (Oncorhynchus tshawytscha), and rainbow trout (Oncorhynchus mykiss) from the south of Chile to the northern hemisphere (Rojas et al., 2009).

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