We present a conditional gene expression system in Saccharomyces cerevisiae which exploits direct RNA-metabolite interactions as a mechanism of genetic control. We inserted preselected tetracycline (tc) binding aptamers into the 5'-UTR of a GFP encoding mRNA. While aptamer insertion generally reduces GFP expression, one group of aptamers displayed an additional, up to 6-fold, decrease in fluorescence upon tc addition. Regulation is observed for aptamers inserted cap-proximal or near the start codon, but is more pronounced from the latter position. Increasing the thermodynamic stability of the aptamer augments regulation but reduces expression of GFP. Decreasing the stability leads to the opposite effect. We defined nucleotides which influence the regulatory properties of the aptamer. Exchanging a nucleotide probably involved in tc binding only influences regulation, while mutations at another position alter expression in the absence of tc, without affecting regulation. Thus, we have developed and characterized a regulatory system which is easy to establish and controlled by a non-toxic, small ligand with good cell permeability.

Riboswitches are newly discovered regulatory elements that consist solely of RNA, sense their ligand in a preformed binding pocket, and perform a conformational switch in response to ligand binding, resulting in altered gene expression. Regulation by a tetracycline (tc)-binding aptamer when inserted into the 5 untranslated region (UTR) of a reporter gene exhibits all characteristics of a riboswitch. Chemical and enzymatic probing reveals that the aptamer consists of two stems, P1 and P2, which are already present in the absence of tc and form the scaffold of the aptamer. They are separated by a bulge B1-2 and an opposing stem-loop (P3-L3). Tc-dependent changes in the probing pattern only appear in the upper part of the bulge B1-2 (nucleotides 9-13) and the loop L3. Saturating mutagenesis corroborates the involvement of these two regions in regulation. Structural probing of the mutant A55U, which contains a single-nucleotide exchange in loop L3 results in a changed probing pattern of the loop, but also of the opposing bulge B1-2. This denotes that both regions cooperate and form a composite binding pocket. Thus, our model for aptamer-mediated translational regulation is that the ligand-free aptamer has only marginal influence on translational initiation. Tc then leads to an intramolecular connection in a pseudoknot-like manner and turns the aptamer into its inhibitory form. This represents a new mechanism for riboswitch action clearly distinguished from currently known naturally occurring riboswitches, which function by sequestration of the ribosomal binding site, transcriptional attenuation, and ribozyme-mediated degradation.

SummaryWe describe post-transcriptional gene regulation in yeast based on direct RNA-ligand interaction. Tetracycline-dependent translational regulation could be imposed via specific aptamers inserted at two different positions in the 5 ¢ ¢ ¢ ¢ untranslated region (5 ¢ ¢ ¢ ¢ UTR). Translation in vivo was suppressed up to ninefold upon addition of tetracycline. Repression via an aptamer located near the start codon (cap-distal) in the 5 ¢ ¢ ¢ ¢ UTR was more effective than repression via a capproximal position. On the other hand, suppression in a cell-free system reached maximally 50-fold and was most effective via a cap-proximal aptamer. Examination of the kinetics of tetracycline-dependent translational inhibition in vitro revealed that preincubation of tetracycline and mRNA before starting translation led not only to the fastest onset of inhibition but also the most effective repression. The differences between the behaviour of the regulatory system in vivo and in vitro are likely to be related to distinct properties of mRNP structure and mRNA accessibility in intact cells as opposed to cell-extracts. Tetracycline-dependent regulation was also observed after insertion of an uORF sequence upstream of the aptamer, indicating that our system also targets reinitiating ribosomes. Polysomal gradient analyses provided insight into the mechanism of regulation. Cap-proximal insertion inhibits binding of the 43S complex to the cap structure whereas start-codon-proximal aptamers interfere with formation of the 80S ribosome, probably by blocking the scanning preinitiation complex.

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