A novel loop-loop recognition motif in the yeast ribosomal protein L30 autoregulatory RNA complex.

The yeast Saccharomyces cerevisiae ribosomal protein L30 negatively autoregulates its production by binding to a helix-loop-helix structure formed in its pre-mRNA and its mRNA. A three-dimensional solution structure of the L30 protein in complex with its regulatory RNA has been solved using NMR spectroscopy. In the complex, the helix-loop-helix RNA adopts a sharply bent conformation at the internal loop region. Unusual RNA features include a purine stack, a reverse Hoogsteen base pair (G11anti-G56syn) and highly distorted backbones. The L30 protein is folded in a three-layer alpha/beta/alpha sandwich topology, and three loops at one end of the sandwich make base-specific contacts with the RNA internal loop. The protein-RNA binding interface is divided into two clusters, including hydrophobic and aromatic stacking interactions centering around G56, and base-specific hydrogen-bonding contacts to A57, G58 and G10-U60 wobble base pair. Both the protein and the RNA exhibit a partially induced fit for binding, where loops in the protein and the internal loop in the RNA become more ordered upon complex formation. The specific interactions formed between loops on L30 and the internal loop on the mRNA constitute a novel loop-loop recognition motif where an intimate RNA-protein interface is formed between regions on both molecules that lack regular secondary structure.