Characterization of YjjJ toxin of Escherichia coli

Reminiscent of eukaryotic apoptotic programmed cell death, bacteria also contain a large number of suicide genes, which are in general co-expressed with their cognate antitoxin genes. These systems called the toxin-antitoxin (TA) systems are associated with cellular dormancy, and play major roles in biofilm formation and persistent multidrug resistance of many human pathogens. In recent years, the study on TA system toxins has become a hot topic due to the health implications of these toxins by virtue of their role in bacterial pathogenicity. Here we report functional characterization of a hitherto uncharacterized Escherichia coli TA toxin, YjjJ. YjjJ exhibits several uncommon properties: (i) unlike the genes encoding most type II TA system toxins, the gene encoding YjjJ is present as a single gene and not in an operon, (ii) despite being a homolog of the well-characterized toxin HipA, YjjJ seems to have different cellular target(s), and (iii) HipB, the cognate antitoxin of HipA, also acts as an antitoxin for YjjJ. This forms a basis for an interesting next step in the study of TA systems with respect to cross-regulation between various TA systems and the evolutionary as well as clinical significance of these observations.