Theophylline-dependent growth. Relative growth curves are shown for C600:ThyA cells containing either Th2P6 (a) and Th1P5 (b) in the presence (black filled circles) and absence (black open circles) of 0.5 mM theophylline or 0.5 mM caffeine (black diamonds). Parental (grey filled squares) and B11 (grey open squares) controls were grown in the 0.5 mM theophylline for comparison. Plots are standardized to the extent of growth of cells containing the parental intron. Each point represents the average of three replicate growth curves. c) Extent of growth at 12 hours for parental, Th2P6 and Th1P5 introns over a range of theophylline concentrations. Background growth (B11) has been subtracted, and results are standardized to parental growth with no theophylline.