Cleavage of supercoiled double‐stranded DNA by several ribosome‐inactivating proteins in vitro

Abstract
Several ribosome‐inactivating proteins (RIPs), such as ricin (including its A‐chain), luffin, cinnamomin and camphorin, were found to express enzymatic activity to cleave supercoiled double‐stranded DNA. In particular, α‐sarcin, a RIP with a novel ribonuclease activity, was first proved to have this activity. They convert supercoiled DNA into a nicked circular conformation at low concentrations and further into a linear form at high concentrations: they have no effect on linear DNA. Although intact type II RIPs exhibited no RNA N‐glucosidase activity, they were detected to cleave supercoiled DNA. Even if ricin A‐chain was treated by boiling, its activity on supercoiled DNA was largely retained.