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A model for cloverleaf tRNA evolution. Three 31-nt minihelices are ligated to form a 93-nt precursor. Two symmetric 9 nt deletions within ligated acceptor stems generate the 75-nt tRNA core. Colors as in Fig. 1. Parentheses indicate base pairing. Asterisks indicate unpaired bases. / indicates a U-turn. / indicates no U-turn.
