Theoretical ligand binding and gene control characteristics of two types of tandem riboswitch architectures. (A) Plot of the fractional aptamer occupancy versus the logarithm of the concentration of ligand for a single aptamer (n = 1) and for two aptamers that are perfectly cooperative (n = 2). The values at the top of the plot report the fold change in ligand concentration needed for the receptor to progress from 10% to 90% receptor occupancy (or gene modulation) as indicated (dashed lines). Curves and values were obtained using the equation provided in the inset (see also Materials and Methods). (B) Plot of the fractional gene control for a single riboswitch (n = 1) and for tandem riboswitches that function independently and exhibit identical K D values (Tandem). Other details are as described for A. (C) Dependence of in vitro transcription termination of B. anthracis tenA mRNA on the concentration of TPP. Theoretical curves for single and tandem riboswitch-mediated transcription termination (as depicted in B) are overlaid to best fit the actual transcription data for comparison. Other details are as described for A.
