An indispensable step in protein biosynthesis is the 2 0 ð3 0 Þ aminoacylation of tRNA by aminoacyl-tRNA synthetases. Here we show that a similar activity exists in a tiny, 5-nt-long RNA enzyme with a 3-nt active center. The small ribozyme initially trans-phenylalanylates a partially complementary 4-nt RNA selectively at its terminal 2 0 -ribose hydroxyl using PheAMP, the natural form for activated amino acid. The initial 2 0 Phe-RNA product can be elaborated into multiple peptidyl-RNAs. Reactions do not require divalent cations, and have limited dependence on monovalent cations. Small size and minimal requirements for regiospecific translational activity strongly support the hypothesis that minuscule RNA enzymes participated in early forms of translation.aminoacyl-RNA | enzyme | evolution | peptidyl-RNA | RNA A mino acids enter modern translation via attachment to a 2 0 ð3 0 Þ tRNA terminus, a reaction catalyzed by a protein aminoacyl-tRNA synthetase. Because it is implausible that primitive peptides were synthesized using already-formed protein catalysts, the RNA world hypothesis (1, 2) requires peptide synthetic reactions performed by RNA enzymes (3, 4). Indeed, a number of RNAs have been isolated which accelerate related translational reactions (5).Several ribozymes capable of catalyzing the same chemical group transfer that is today carried out by aminoacyl-tRNA synthetases (6-9) have been isolated using Systematic Evolution of Ligands by Exponential Enrichment (10, 11). However, none possess all desirable characteristics. First, an RNA world enzyme should be small, accessible after rudimentary RNA synthesis. In addition, it should act in trans, and should use universal biological, water-soluble substrates. Previously isolated ribozymes employ appropriate substrates [amino acids activated as acyladenylates (6, 12)], but are not true enzymes, as they are self-aminoacylators modified by their own reaction. Other ribozymes aminoacylate RNA with turnover; however, the amino acids must be activated as cyanomethyl, 3,5-dinitrobenzyl, or p-chlorobenzyl esters (8, 13), and hence they do not facilitate the biological reaction.The family of self-aminoacylating ribozymes exemplified by truncate C3 RNA (Fig. 1A) presented the intriguing possibility of very simple aminoacyl transfer (6). Mutational analyses as well as molecular dynamics and energy minimization of the reactants suggested a tiny active center consisting of only three essential nucleotides-a 3 0 -terminal U, and a 5 0 -GU-3 0 sequence in a loop apposed to the unpaired 3 0 -terminal U. Although the C3 RNA family possessed helical elements, adjacent helices appeared nonspecific in sequence, perhaps required only for assembly of the active center (6).Here we present a radically modified version of C3 ribozyme, unique in three ways: It functions in trans; it has been minimized to a tiny, five-nucleotide ribozyme; and it also supports peptidyl-RNA synthesis. The result is the smallest trans-aminoacylator, and arguably the smallest true ribozyme, ever observed (...

Selection-amplification finds new RNA enzymes (ribozymes) among randomized RNAs with flanking unvaried sequences (primer complements). Precise removal of 3′-primer before reaction selected aminoacylation from PheAMP in 3 cycles, yielding active RNAs (k cat = 12-20 min -1 ) using only three conserved nucleotides, acting independently of divalent ions. This unusually simple RNA active site encouraged study of the reaction via molecular mechanics-based free energy minimization. On this basis, we suggest a chemical path for RNA-catalyzed transaminoacylation. Site modeling also predicted new features -L-stereoselectivity, 2′-regioselectivity, independence of amino acid side chain and phosphorylated activating group, that were subsequently verified. The same selection also showed that RNA aminoacylation from adenylate is simpler than from CoA thioester, potentially rationalizing translational activation by adenylates. The simplicity of this active site suggests a general route to small ribozymes.

[reaction: see text]. beta-Hydroxy sulfone-based tethers were employed for the first time to achieve thermally mediated intramolecular Diels-Alder cycloaddition. The reactions proceeded with complete regioselectivity and high (10/1) to complete endo/exo-selectivity and resulted in the preferential formation of one of the two possible endo-cycloadducts. The yields and stereoselectivities were proportional to the bulk of the R(1) substituent on the beta-acyloxysulfonyl tether.