Small-molecule-dependent pre-mRNA splicing. The IgM-MS2 pre-mRNA was incubated in the presence or absence of MS2-RS9G8, FRB*-RS9G8, FRB*-RSSC35, MS2-F3, rapamycin*, or FK506 as indicated and resolved on either a denaturing polyacrylamide gel (A) or a native polyacrylamide gel (B). The concentrations of rapamycin* and FK506 used in the experiments are as follows: (A, lanes 6,9) 200 nM rapamycin*; (B, lane 3) 200 nM rapamycin*, (lane 4) 500 nM rapamycin*, (lane 5) 5 μM FK506 and 500 nM rapamycin*; (C, lane 6) 500 nM rapamycin*, (lane 7) 5 μM FK506 and 500 nM rapamycin*.