Positive-strand RNA viruses direct different virus-specific processes during their infection of host cells. Fundamental events such as viral RNA genome replication are controlled by viral regulatory RNA elements (REs). Here, we have investigated the possibility of specifically modulating the action of a viral RE using RNA aptamer technology. Through rational design, a tombusvirus RE, which has the structure of a perfect RNA stem loop in the plus-strand RNA genome, was replaced with a theophylline-binding RNA aptamer sequence, an imperfect stem loop. The aptamer-RE hybrid was designed so that, upon binding theophylline, it would become more stable and structurally mimic the functional RE (i.e., represent a ligand-inducible RE riboswitch). Initial experiments were conducted with a small noncoding virus genome-derived RNA replicon, and the results showed that replication was inducible, up to Ϸ10-fold, in a theophylline-specific and dose-dependent manner. A similar level of theophylline-dependent induction was also observed when a full-length viral genome containing an RE riboswitch was tested. Analysis of this engineered viral genome revealed that this RE, located in the 5 untranslated region, specifically mediates efficient accumulation of plus-strands of the virus genome. Therefore, in addition to allowing for modulation of virus reproduction, the RE riboswitch system also provided insight into RE function. The ability to chemically induce a viral process via modulation of virus genome structure could be useful for basic and applied aspects of research.aptamer ͉ gene regulation ͉ riboswitch ͉ RNA structure ͉ tombusvirus R NA aptamers are RNA molecules that are capable of specifically binding to cognate ligands. Small moleculebinding RNA aptamers were first identified through artificial selection by using systematic evolution of ligands by exponential enrichment (i.e., SELEX) (1). Surprisingly, some of these RNA aptamers showed favorable binding affinities and high levels of specificity (1). These observations suggested that natural RNA aptamers might exist and function in nature. Indeed, naturally occurring RNA aptamers, present as components of riboswitches, were subsequently identified in bacteria and shown to be involved in regulating translation or transcription of cellular mRNA (2). These riboswitches contain two mutually exclusive structures, one functional and the other nonfunctional, and ligand binding to the aptamer domain within the riboswitch acts to stabilize one of the two structures (2). In this way, the presence or absence of the ligand dictates which RNA structure dominates and, thus, the activity of the functional RNA module.Engineered RNA-based regulation of gene expression in living cells is now a rapidly expanding area of biotechnology. RNA aptamers that bind to small-molecule ligands (e.g., theophylline) have been instrumental in the development of novel systems that allow for ligand-dependent regulation of mRNA translational activity (3-10), ribozyme activity (10, 11), or other cellular proc...