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Ribosome-inactivating proteins (RIPs) were demonstrated to exhibit a unique enzymatic activity on cleaving supercoiled double-stranded DNA into the nicked or linear form. Although there is an interaction between supercoiled DNA and RIP, no sequence-specific recognition was involved. Instead, RIPs recognize supercoiled DNA by conformational specificity. Negatively supercoiled DNA is the preferential conformation in the action of RIPs. When double-stranded DNA occurs in the supercoiled form, even if with lower linking number, RIPs can still convert it into nicked or linear form. Terminal-labelling experiments indicated that radioactivity was incorporated into putative 5'-ends of nicked or linear DNA generated by RIPs. We conclude that RIPs act as a novel supercoil-dependent endonuclease when they cleavage supercoiled DNA. The impossibility that contaminating enzymes in the RIP preparations cleaved the supercoiled DNA is briefly discussed.

Topological Requirements for Recognition and Cleavage of DNA by Ribosome-Inactivating Proteins