Targeted cleavage: Tuneable cis-cleaving ribozymes

Posttranscriptional regulation of gene expression has become a popular method for studying gene function and elucidating networks of gene expression. A number of tools are available that allow investigators to regulate gene expression posttranscriptionally, including RNAi, antisense oligonucleotides, DNAzymes, and ribozymes (1). Each of these methods relies on complementary basepairing between the inhibitory agent and the target mRNA. In addition, these methods require efficient delivery of short nucleic acids to cells through either carrier molecules or the introduction of genes that encode the inhibitory molecules for expression in the cells of interest (1). A unique nucleic acid inhibitor must be developed for every target of interest and, in many instances, a panel of the appropriate nucleic acid inhibitors has to be tested to achieve the desired knockdown levels of the target (2, 3).

In a recent issue of PNAS, Win and Smolke (4) describe a novel platform for posttranscriptional regulation of gene expression. Their system uses allosterically regulated, cis-cleaving ribozymes. Their system is devised such that cleavage takes place within the 3′ UTRs of the mRNAs. The premise is that cis cleavage results in separation of the poly(A) tail from the body of the message, and deadenylation is an initiating event for degradation of mRNAs (5). The system of Win and Smolke is an excellent example of the application engineering principles to a biological system. Their goals were to create a portable, scaleable, tuneable platform for regulation of gene expression.

To better understand the system of Win and Smolke (4), one needs to look at each of the components. The foundation for their regulatory system is the hammerhead ribozyme, initially discovered as a cis-cleaving entity in single-stranded viroid and virusoid RNAs (6). Although the hammerhead ribozyme has been the object of extensive studies and applications over the past …

*E-mail: jrossi{at}coh.org