In order to determine the parameters that govern the activity of a ribozyme in vivo, we made a systematic analysis of chimeric tRNAVal ribozymes by measuring their cleavage activities in vitro as well as the steady-state levels of transcripts, the half-lives of transcribed tRNAVal ribozymes, and their activities in both HeLa and H9 cells. These analyses were conducted by the use of transient expression systems in HeLa cells and stable transformants that express ribozymes. Localization of

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... ipts appeared to be determined by the higher-order structure of each transcribed tRNAVal ribozyme. Since colocalization of the ribozyme with its target RNA is important for strong activity of the ribozyme in vivo, the best system for tRNA-based expression seems to be one in which the structure of the transcript is different from that of the natural tRNA precursor so that processing of the tRNAValribozyme can be avoided. At the same time, the structure of the transcript must be similar enough to allow recognition, probably by an export receptor, so that the transcript can be exported to the cytoplasm to ensure colocalization with its target. In the case of several tRNAVal ribozymes that we constructed, inspection of computer-predicted secondary structures enabled us to control the export of transcripts. We found that only a ribozyme that was transcribed at a high level and that had a sufficiently long half-life, within cells, had significant activity when used to withstand a challenge by human immunodeficiency virus type 1.