Fig. 1. Assembly of the violacein expression cassette and its application in Pb(ii) biosensing. (A) The design of the pigment-based whole-cell biosensors. Circuit diagram and schematic depicting the assembly of the violacein biosynthetic module vioABCDE under the control of T7 lac promoter (top). The DNA sequence containing the stop codon of the upstream gene, an extra RBS, and the start codon of the downstream gene is shown. Genetic organization of the lead (bottom) inductive violacein expression constructs. The Pb(ii) sensing element (the pbrR gene and its divergent promoter) was inserted in front of the vioABCDE to generate the lead biosensing module. (B) A schematic of the workflow envisioned for the implementation of the colorimetric biosensors towards Pb(ii).