has direct part weakly interacting with boundary of has boundary family 9 base pair 2 A family_9_base_pair is an RNAO_base_pair in which two RNA nucleotides interact by edge-to-edge hydrogen-bonding between the Hoogsteen edge of one base and the Sugar edge of the other base and in which the glycosidic bonds of the two nucleotides are oriented "cis" relative to each other. nucleobase ribonucleoside residue Hoogsteen edge sugar edge cis-oriented base pair