Specific recognition of metabolites by functional RNA motifs within mRNAs has emerged as a crucial regulatory strategy for feedback control of biochemical reactions. Such riboswitches have been demonstrated to regulate different gene expression processes, including transcriptional termination and translational initiation in prokaryotic cells, as well as splicing in eukaryotic cells. The regulatory process is usually mediated by modulating the accessibility of specific sequence information of the expression platforms via metabolite-induced RNA conformational rearrangement. In eukaryotic systems, viral and the more limited number of cellular decoding -1 programmed ribosomal frameshifting (PRF) are commonly promoted by a 3' mRNA pseudoknot. In addition, such -1 PRF is generally constitutive rather than being regulatory, and usually results in a fixed ratio of products. We report here an RNA pseudoknot capable of stimulating -1 PRF whose efficiency can be tuned in response to the concentration of S-adenosylhomocysteine (SAH), and the improvement of its frameshifting efficiency by RNA engineering. In addition to providing an alternative approach for small-molecule regulation of gene expression in eukaryotic cells, such a metabolite-responsive pseudoknot suggests a plausible mechanism for metabolite-driven translational regulation of gene expression in eukaryotic systems.