Scheme 2.

Representation of a ‘classical’ U-turn (left) (25) and the U-turn in the ASL of S.pombe met-tRNAi (right) on left and right panels, respectively. R represents a purine and N a purine or pyrimidine nucleobase. Stabilizing hydrogen bonding interactions of the U-imino with 5′ phosphate following the UNR sequence and of 2′ hydroxyl of U with R:N7 are indicated in red dashed lines. Atoms U33:O2' and A35:N7 that are within hydrogen bonding distance are also connected by a red dashed line.