Inhibition of prokaryotic translation by the Shiga toxin enzymatic subunit

Shiga toxin, produced by Shigella dysenteriae serotype 1, is a member of the large family of ribosome-inactivating proteins (RIPs) which are primarily produced by plants. All RIPs are rRNA N-glycosidases which inactivate ribosomes through the removal of a specific adenine residue from the well-conserved aminoacyl-tRNA-accepting loop of rRNA. As a type II RIP, STX is believed to have little effect on prokaryotic ribosomes. However, we have demonstrated that over-expression of the STX enzymatic (A1) polypeptide which lacks a signal sequence caused a reduced rate of growth of its Escherichia coli host. Over-expression of the same StxA1 polypeptide with a catalytic site substitution had no effect on the growth of E. coli. In addition, purified StxA1 was an inhibitor of prokaryotic protein synthesis as assessed using an in vitro transcription and translation assay. The specific activity of StxA1 was significantly higher than ricin, which is another type II RIP, with both eukaryotic and prokaryotic translation systems.