FMRP Specifically Binds Sod1 mRNAFMRP binding to Sod1 mRNA is not dependent on K+. Labeled G-quartet RNA (N19) or Sod1 full-length mRNA were incubated with increasing amounts of recombinant His-FMRP in the presence of K+(A) or Na+(B). FMRP/Sod1 binding was not affected by ionic conditions, whereas, as expected, the presence of Na+ affected FMRP binding to N19.(C) Gel-shift experiments were performed using a 32P-labeled N19 probe incubated with 0.1 pmol of recombinant His-tagged FMRP in the presence of increasing amounts of unlabeled competitors, ranging from 10−9 to 10−7M [lanes 3–5 (N19), lanes 6–8 (Sod1), lanes 9–11 (N8)]. Lane 1, no protein control; lane 2, no competitor control. Note that both N19 (positive control) and Sod1 compete equally well for binding to FMRP, whereas N8 (negative control) only competes at high concentrations (nonspecific binding). All data obtained in these experiments are listed in Table S2.