A diminutive and specific RNA binding site for L-tryptophan

Selection for amino acid affinity by elution of RNAs from tryptophan–Sepharose using free L-tryptophan evokes one sequence predominantly (K(D) = 12 µM), a symmetrical internal loop of 3 nt per side. Though we have also isolated larger sequences with affinity for tryptophan, successively squeezed selection in randomized tracts of 70, 60, 40, 20 and 17 nt show that this internal loop is the simplest sequence that can meet the column affinity selection. From sequence variation in ∼50 independent isolates, only 26 bits of information are required to describe this loop (equivalent to only 13 fully conserved nucleotides). Thus, it is among the simplest amino acid binding sites known, as well as selective among hydrophobic side chains. Among site sequences defined as essential to affinity by conservation, protection and modification-interference, there is a recurring CCA sequence (a tryptophan anticodon triplet) which apparently forms one side of the binding site. Such conserved juxtaposition of tryptophan with a cognate coding triplet supports a stereochemical origin for the genetic code.