Factors governing the activity in vivo of ribozymes transcribed by RNA polymerase III

In order to determine the parameters that govern the activity of a ribozyme in vivo, we made a systematic analysis of chimeric tRNA(Val) ribozymes by measuring their cleavage activities in vitro as well as the steady-state le vels of transcripts, the half-lives of transcribed tRNA(Val) 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 transfo rmants that express ribozymes. Localization of transcripts appeared to be d etermined by the higher-order structure of each transcribed tRNA(Val) riboz yme. Since colocalization of the ribozyme,vith 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 dif ferent from that of the natural tRNA precursor so that processing of the tR NA(Val) ribozyme can be avoided. At the same time, the structure of the tra nscript 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 tRNA(Val) ribozymes that we constructed, inspection of computer-predicted secondary structures enabled us to control the export of transcripts. We found that only a ribo zyme 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.