Coaxial helix

Two separate helical regions stack to form coaxial helices as a pseudo-continuous (quasi-continuous) helix. Coaxial helices are highly stabilizing tertiary interactions and are seen in several large RNA structures, including tRNA, pseudoknots, the group I intron P4-P6 domain, and in the Hepatitis Delta Virus ribozyme.

Reference: Kim,S.H., J.L.Sussman, F.L.Suddath, G.J.Quigley, A.Mcpherson, A.H.J.Wang, N.C.Seeman, and A.Rich. 1974. General Structure of Transfer-Rna Molecules. PNAS USA 71:4970-4974.