3-Methylxanthine dependent growth. a) Relative growth curves are shown for C600:ThyA cells containing 3MeX2P6 in the presence (black filled circles) and absence (black open circles) of 1 mM 3-methlyxanthine or 1 mM theophylline (black diamonds). Parental (grey filled squares) and B11 (grey open squares) controls were also grown in 1 mM 3-methylxanthine. Plots are standardized to the extent of growth of cells containing the parental intron. Each point represents the average of three replicate growth curves. b) Specificity of growth of 3MeX2P6 on theophylline analogs. Background growth (B11) has been subtracted, and results are standardized to parental growth on minimal media at 12 hrs. Negative values were not greater than -0.05, and were set to zero. Each analog was present in the media at 0.5 mM. c) Specificity of growth of Th2P6 on theophylline analogs. Presentation of the data is the same as in Figure 6b. Each analog was present in the media at 0.5 mM.