Abstract
Fifty years ago, the first atomic-resolution structure of a nucleic acid double helix, the mini-duplex (ApU)(2), revealed details of basepair geometry, stacking, sugar conformation, and backbone torsion angles, thereby superseding earlier models based on x-ray fiber diffraction, including the original DNA double helix proposed by Watson and Crick. Just 3 years later, in 1976, Ned Seeman, John Rosenberg, and Alex Rich leapt from their structures of mini-duplexes and H-bonding motifs between bases in small-molecule structures and transfer RNA to predicting how proteins could sequence specifically recognize double helix nucleic acids. They proposed interactions between amino acid side chains and nucleobases mediated by two hydrogen bonds in the major or minor grooves. One of these, the arginine-guanine pair, emerged as the most favored amino acid-base interaction in experimental structures of protein-nucleic acid complexes determined since 1986. In this brief review we revisit the pioneering work by Seeman et al. and discuss the importance of the arginine-guanine pairing motif.