Oligonucleotide Synthesis.

RNA oligonucleotides were synthesized using 2′-O-triisopropylsilyloxymethyl–protected phosphoramidites (Chemgenes). DNA bases were added as standard cyanoethyl phosphoamidites (Glen Research). For thiol modification of oligonucleotides, 1-O-dimethoxytrityl-propyl-disulfide,1′-succinyl-lcaa-controlled pore glass beads or S-trityl-6-mercaptohexyl-1-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite were used for 3′ and 5′ end modification, respectively (Glen Research). For spacer 18 modification, 18-O-dimethoxytritylhexaethyleneglycol,1-[(2-cyanoethyl)-(N,N-diisopropyl)]-phosphoramidite was used (Glen Research). The 5′ modified oligonucleotides were synthesized on 1,000-Å UnyLinker solid support (Chemgenes). Oligonucleotides were synthesized on a MerMaid 12 system and deprotected as recommended by the manufacturers (Bioautomation). Deprotected oligonucleotides were purified with reverse-phase high-performance liquid chromatography (HPLC) on a C18 column using 0.1 M triethylammonium acetate and acetonitrile as the solvent system. After HPLC, the 5′-DMT group was removed in 20% acetic acid at room temperature for 1 h, and the solution was extracted 3 times with ethyl acetate. The aqueous phase of this solution was lyophilized and resuspended in sterile water. The oligonucleotide sequences used in this work are listed in SI Appendix, Table S1.