Final answer:
The aptamer and expression platform are the two important domains within a riboswitch involved in ligand binding and gene expression regulation.
Step-by-step explanation:
The two important domains within a riboswitch that involve the ligand-binding site are the aptamer and the expression platform. The aptamer is a structured domain of the riboswitch RNA molecule that directly binds to the ligand, and the conformation of this domain changes upon ligand binding. The expression platform is the part of the riboswitch which transduces the binding event into a change in gene expression, often by undergoing a structural rearrangement that affects the RNA's functionality.
The two important domains within a riboswitch that involve the ligand-binding site are the aptamer and expression platform.