oligoribonucleotides were synthesized on an mm12 synthesizer ( bioautomation corp . , irving , tx ) on a 50  nmol scale with 2otbdmsphosphoramidites ( thermo fisher ) , using a 1000   unylinker support ( chemgenes , wilmington , ma ) . coupling time was 290  s for regular 2otbdms phosphoramidites , 2180  s for the dnb phosphoramidite 5 , and 3240  s for 2opropargyl rc phosphoramidite . isolated 5dnbprotected oligonucleotides were dried in a speedvac and redissolved in 200  l rnasefree water prior to irradiation for 10  min at 365  nm in a vilber lourmat biolink blx crosslinker ( 58  w ) . final rphplc purification yielded 5phosphate oligoribonucleotides ready for the introduction of functional groups by click chemistry ( e.g. , psoralen , cy5 ) , and for enzymatic ligation experiments with t4  rna ligase  2 ( m0239 , new england biolabs ) ( see the supporting information for details ) . as a service to our authors and readers , this journal provides supporting information supplied by the authors . such materials are peer reviewed and may be reorganized for online delivery , but are not copyedited or typeset . technical support issues arising from supporting information ( other than missing files ) should be addressed to the authors .