Fig. 1
The combination of the novel theophylline-dependent riboswitch E1 and the P43 promoter in Bacillus subtilis. a Diagrams of the expression cassette driven by the native P43 promoter and the novel theophylline-dependent expression element (P43′-riboE1). The green fluorescent protein is represented by the green arrows, and the transcription start site is marked as TSS. The promoter fragment of the novel expression element, P43′, was produced by deletion of the entire sequence downstream of the native P43. The synthetic riboswitch E1 was placed immediately downstream of P43′, resulting in the novel expression element P43′-riboE1. The riboswitch was derived from the riboswitch E (previously reported by Topp et al.), in which a single A was inserted before the 5′ terminus of the original sequence. In the diagram, the inserted nucleotide is coloured red. The dash line denotes the difference between native P43 and the synthetic riboswitch E1 (riboE1). b The schematic diagram of mechanism of the novel genetic element. Theophylline is indicted in blue. c The growth curves of B. subtilis 168 harbouring pP43-gfp (BSG43) and pBSG11 BSG11). The recombinant B. subtilis strains were inoculated by pre-cultures with the initial OD600 of 0.05. The cultures were sampled periodically to measure the cell density until the cell density began to decrease at 27 h. d Expression levels of GFP (y-axis) against time (x-axis) during the culture period. The dashed line denotes the induction by 4 mM theophylline. The GFP fluorescence was measured in triplicates and the data were shown in mean ± SD