Theophylline-dependent td group I intron constructs. a) The figure shows the predicted secondary structure and tertiary interactions of the 265 nt deletion construct[15, 41] of the td intron drawn in the format of Cech et al. (1994) The intron is in uppercase and the exons are in lower case letters. The 5' and 3' splice sites are indicated by arrows. The P4-P6 domain is boxed. b) The B11 construct was based on the Δ 85–863 deletion mutant of the td intron which showed no activity in vivo or at low Mg2+ (3 mM) in vitro. An anti-theophylline aptamer, highlighted in gray, was substituted for the P6a stem of the intron in constructs Th1P6, Th2P6, Th3P6, Th4P6, Th5P6 and Th6P6, and for the P5 stem in constructs Th1P5 and Th2P5. Mutations in the anti-theophylline aptamer are boxed in black for constructs MeX2P6 and Th2P6.D. The C-to-A mutation in MeX2P6 changes specificity from theophylline to 3-methylxanthine . The A-to-U and C-to-U mutations in Th2P6.D abolish theophylline-binding [11, 43].