Figure 6.

Principle of functional/topological equivalence. The kink turn [Right: in H5-7 domain (PDB_ID: 1JJ2)] and UA_h turn [Left: in H82–87 domain (PDB_ID:2J01)] can both be components of the doughnut-PK domain identified in the 23S rRNA. Albeit of different sequence signatures and local structures, they are topologically equivalent as they both contribute to the folding of the same topological domain. The doughnut-PK domain can act as scaffolding for structural expansion by accommodating sequence insertions in the UA_h turn (left). For annotations see Figures 2 and S3.