Topological requirements for recognition and cleavage of DNA by ribosome-inactivating proteins

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.