Citron1 is a genetically-encoded biosensor for citrate. It contains residues 4-133 of the periplasmic citrate-binding (CitAP) domain of Klebsiella pneumoniae CitA protein inserted into GFP via two linkers. (Following optimization, there are 14 mutations in GFP and 8 mutations in CitAP.) Binding of citrate to CitAP is communicated to GFP to allosterically control the chromophore environment and increase its fluorescent brightness. (PMID:32875085.)