Monitoring of mercury uptake in bacterial cells. The time-dependent response curve was obtained by measuring the FRET ratio after the incubation of bacterial cell suspension containing MerFS-51 with 0.5 µM mercury. 0.5 µM Hg²⁺ was also added to MerFS-control and the ratiometric change was obtained for the defined period. Data are means of three independent replicates (n = 3). Vertical bars indicate standard error