FIGURE 4.

The fluoride riboswitch modulation is affected by media and host strain. (A) Fold induction (FI) of total luciferase activity in cultures was calculated over an incubation period of ∼ 40 h (B. thailandensis transformed with the B. thailandensis E264 fluoride riboswitch construct—pVK-f2-lux) and ∼55 h (B. thailandensis E264, transformed with P. syringae fluoride riboswitch construct—pVK-f-lux). Both plasmids are replicative in E. coli strains and integrative in B. thailandensis E264. Both pre-culture media (LB) and expression media (0.5X M9-MM) were supplemented with appropriate antibiotics depending on the species (see Methods). (B) The same results are represented with the direct luminescence (sum) data. (C) Fluoride induction concentrations were 16 mM for E. coli DH5α and E. coli SM10λpir transformants and 31 mM for B. thailandensis E264. Concentrations were chosen according to the maximum induction effect. The FI values for each sample represent the means and standard deviations of triplicates on the same microplate. (D) Fold induction (FI) of B. thailandensis E264 transformed with the B. thailandensis E264 fluoride riboswitch for 31 mM and 62 mM of fluoride. (E–G) Different culturing conditions (MgCl₂ vs MgSO₄) were also evaluated. A much stronger induction by fluoride can be noticed with MgCl₂.