The crystal structure of a 26-nucleotide RNA containing a hook-turn.

A crystal structure has been obtained for a 26-nucleotide RNA that contains the loop E sequence from Chromatium minutissimum. Rather than having a loop E-like conformation, it consists of an A-form helix that splits into two separate strands following a sheared A-G base pair. The backbone of the strand containing the G of the A-G pair makes a turn of almost 180 degrees in the space of two nucleotides, and then interacts with the minor groove of the helix from which it originates. Similar structures, which we call hook-turns, occur in 16S and 23S rRNAs. They are found at places where the two strands of a helix separate at an A/G juxtaposition to interact with other sequences.