Metabolite sensors reporting on cytosolic arginine, serine, and O -acetyl-serine. Cells of E. coli DH5α (left) carrying pSenArg fluoresce when Arg-Ala was added (top), but not following supplementation with Ala-Ala (bottom). The peptide-dose response curves for Arg-Ala, Lys-Ala and His-Ala are shown in Figure S3 of Additional file 1. The serine-producing strain C. glutamicum-Ser4 (middle)  with pSenSer is fluorescent (top), but the WT carrying pSenSer is not (bottom). (c) Fluorescence is seen with the L-cysteine producer C. glutamicum-Cys3 carrying pSenOAS (top right), but not with the control strain (bottom right). Epifluorescence microscopic analysis was done at λex = 490 to 510 nm and λem = 520 to 550 nm.