The effect of the protein synthesis inhibitor, cycloheximide (40 μg/mL), on (A) ³H-leucine incorporation into protein over 1 to 20 hours and (B) on the RNA-binding activity of the IRPs during 1 to 20 hours of incubation with control medium (control) or 311 (25 μmol/L). The result in (A) is the mean ± SD of 5 replicates in a typical experiment from two experiments performed. The result in (B) is a typical experiment from four experiments performed.