Ribozyme mediated destruction of RNA in vivo.

Previous studies have demonstrated that high ribozyme to substrate ratios are required for ribozyme inhibitory function in nuclear extracts. To obtain high intracellular levels of ribozymes, tRNA genes, known to be highly expressed in most tissues, have been modified for use as ribozyme expression cassettes. Ribozyme coding sequences were placed between the A and the B box, internal promoter sequences of a Xenopus tRNAMet gene. When injected into the nucleus of frog oocytes, the ribozyme tRNA gene (ribtDNA) produces 'hammerhead' ribozymes which cleave the 5' sequences of U7snRNA, its target substrate, with high efficiency in vitro. Oocytes were coinjected with ribtDNA, U7snRNA and control substrate RNA devoid of a cleavage sequence. It was found that the ribtRNA remained localized mainly in the nucleus, whereas the substrate and the control RNA exited rapidly into the cytoplasm. However, sufficient ribtRNA migrated into the cytoplasm to cleave, and destroy, the U7snRNA. Thus, the action of targeted 'hammerhead' ribozymes in vivo is demonstrated.