The cis-encoded antisense RNA IsrA from Salmonella Typhimurium represses the expression of STM0294.1n (iasE), an SOS-induced gene coding for an endoribonuclease activity.

Toxin-antitoxin systems are known to be involved in many bacterial functions that can lead to growth arrest and cell death in response to stress. Typically, toxin and antitoxin genes of type I systems are located in opposite strands, where the antitoxin is a small antisense RNA (sRNA). In the present work we show that the sRNA IsrA from Salmonella Typhimurium down-regulates the expression of its overlapping gene STM0294.1n. Multiple sequence alignment and comparative structure analysis indicated that STM0294.1n belongs to the SymE toxin superfamily, and the gene was renamed iasE (IsrA-overlapping gene with similarity to SymE). The iasE expression was induced in response to mitomycin C, an SOS-inducing agent; conversely, IsrA overexpression repressed the iasE expression even in the presence of mitomycin C. Accordingly, the inactivation of IsrA with an anti-IsrA RNA expressed in trans abrogated the repressive effect of IsrA on the iasE expression. On the other hand, iasE overexpression, as well as the blockage of the antisense IsrA function, negatively affected bacterial growth, arguing for a toxic effect of the iasE gene product. Besides, a bacterial lysate obtained from the iasE-overexpressing strain exhibited endoribonuclease activity, as determined by a fluorometric assay based on fluorescent reporter RNAs. Together, these results indicate that the IasE/IsrA pair of S. Typhimurium constitutes a functional type I toxin-antitoxin system.