The aptamers were transcribed in vitro from a linearized pSP64 plasmid (Promega) using a T7 promoter. For precise aptamer 3′ ends the primary transcripts contained self-cleaving hammerhead ribozymes (all sequences are available upon request). Run-off transcription was done in a volume of 2 ml containing 40 mM Tris-HCl (pH 8.0), 5 mM DTT, 1 mM spermidine, 30 mM Mg-acetate, 4 mM each NTP, 0.1 mg/ml HindIII-linearized plasmid and 15 µg/ml T7 RNA polymerase [prepared according (14 (link))]. Reactions were incubated at 37°C for 4 h, subsequently diluted 1:5 in 30 mM Mg-acetate and subjected to two annealing cycles (5 min at 65°C, followed by slow-cooling to room temperature over 45 min) in order to maximize self cleavage of the ribozyme. After dissolving precipitated Mg-pyrophosphate by dropwise addition of 0.5 M EDTA (pH 8.0), the reaction products were pre-purified on 1 ml pre-equilibrated DEAE–Sepharose FF anion exchange resin [Pharmacia®, 0.7 cm column diameter, equilibration in 300 mM, elution in 3 M Na-acetate (pH 7.5)]. After ethanol-precipitation, the RNA products were separated on a 10% denaturing polyacrylamide gel. Aptamer RNA was detected by ultraviolet (UV) shadowing and eluted from crushed gel slices in 300 mM Na-acetate over night at room temperature. The supernatant was again concentrated on DEAE–Sepharose and ethanol precipitated. RNA was resuspended in H2O (0.1–10 mg/ml) and stored frozen at −20°C. Prior to complex formation the RNA was adjusted to the respective buffer system and annealed (see above). Complexation with tc was assured by an additional co-incubation for 5 min at 37°C.