Fig. 1. Repressible chloroplast gene expression system. Scheme of the repressible chloroplast gene expression system. The nuclear Nac2 gene is fused either to the Cyc6 promoter which is repressed by copper or the MetE promoter and Thi4 riboswitch which are repressed by vitamin B12 and thiamine, respectively. The target of Nac2 is the psbD 5′UTR. In this particular case it was fused to the coding sequence of rps12 making the expression of this gene dependent on Nac2 (strain RR5). The coding sequence of psbD was fused to the psaA 5′UTR to render psbD expression independent of Nac2 [8].