has direct part weakly interacting with boundary of has boundary canonical RNA base pair 2 An RNAO_base_pair which has cis-Watson--Crick pairing and where the participating bases are AU, CG, GC or UA. ribonucleoside residue cytosine nucleobase uracil nucleobase adenine nucleobase guanine nucleobase Watson-Crick edge cis-oriented base pair