Which of the following is NOT a feature of riboswitches?

Riboswitches are unique regulatory elements found in the non-coding regions of messenger RNA (mRNA) that control gene expression in response to specific metabolites or ions. One of their key characteristics is that they change conformation upon binding to a ligand, which allows them to influence processes such as transcription termination and translation initiation.

Looking at the provided choices, the statement about riboswitches being large protein-coding sequences is incorrect as riboswitches are primarily composed of RNA, not protein. They are typically short segments of RNA that can fold into specific three-dimensional structures to interact with small molecules, not long sequences associated with protein-coding.

Additionally, riboswitches can affect transcription (by forming terminator structures) and can also play a role in gene regulation by influencing the accessibility of ribosome binding sites, making them critical for cellular regulation. Thus, identifying the feature that does not align with the characteristics of riboswitches leads to the conclusion that they are not associated with being large protein-coding sequences.