The invention provides polynucleotide constructs for the regulation of gene expression by aptamer-based modulation of the accessibility of one or more polyadenylation signals and methods of using the constructs to regulate gene expression in response to the presence or absence of a ligand that binds the aptamer. The polynucleotide construct contains a riboswitch comprising an aptamer and an effector stem loop, wherein the effector stem loop comprises a polyadenylation signal sequence.
