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 minutis-simum. 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-C base pair. The backbone of the strand containing the G of the A-G pair makes a turn of almost 180 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.