a) The conformation of transcription factor (TF, orange oval) changes to active form upon binding the target ligand (blue octagon). When activated, the TF binds to its binding site (light orange square), upstream of a fluorescent reporter gene, to induce production of a fluorescent reporter protein (green oval) that is detected by flow cytometry.
b) FRET sensors comprised of a donor-acceptor fluorophore pair. Ligand is sandwiched between the two donor (orange cylinder) and acceptor fluorophores (green cylinder). Therefore, a conformation of FRET is changed that allows detecting the fluorescent signal by flow cytometry.

c) Correctly folded aptamer structure of riboswitch (orange–gray structure) allows transcription of fluorescent reporter gene (green arrow). The production of fluorescent protein (green oval) is detected by flow cytometry. In presence of ligand (blue octagon), the secondary structure of riboswitch device is changed. Consequently, transcription of its fluorescent reporter gene is inhibited.[1]
https://doi.org/10.1038/s41467-020-16175-y
Gita Naseri &amp; Mattheos A. G. Koffas