Essential structures of a self-aminoacylating RNA.

Comparison of six independent self-aminoacylating RNAs derived from selection-amplification, as well as deletion, addition, substitution, fragmentation of one particular RNA, are used to analyze the requirements for the RNA-catalyzed aminoacylation. All elements required for catalysis by one RNA family: sequence at the 3' acceptor end, calcium and magnesium sites, as well as the Phe-AMP substrate site and the essential 5' triphosphate terminus, are closely grouped near a bihelix junction in the parental molecule. All elements of the active center for aminoacyl transfer can therefore be captured by a peripherally-deleted helix junction RNA, defining a much smaller 43 nucleotide ribozyme, of which only 17 nucleotides were initially randomized. It appears that a complex RNA active center can be assembled by specifying unexpectedly few nucleotides, perhaps with a critical contribution from an essential calcium ion.