Endoribonuclease active on single-stranded mRNA. Inhibits protein synthesis by cleavage of mRNA. Previously thought to inhibit protein synthesis initiation. This protein may also be involved in the regulation of purine biosynthesis. YjgF (renamed RidA) family members are enamine/imine deaminases. They hydrolyze reactive intermediates released by PLP-dependent enzymes, including threonine dehydratase. YjgF also prevents inhibition of transaminase B (IlvE) in Salmonella.