Richard S. A. Lipman

A tRNA circularization assay: evidence for the variation of the conformation of the CCA end.

The CCA end is common to all tRNAs as the universal site for amino acid attachment+ It is also conserved in the 39-terminal tRNA-like structure of viral genomes that can be aminoacylated by an aminoacyl-tRNA synthetase (Florentz & Giege, 1995)+ During aminoacylation, the CCA end enters the catalytic center of an aminoacyl-tRNA synthetase and provides the site for chemistry to take place+ The CCA end is also widely used in replication of retroviruses, the bacterial singlestranded RNA viruses, and duplex DNA plasmids of fungal mitochondria+ During replication, the CCA end interacts with the template-specificity domain of reverse transcriptase or replicase and provides the initiation site for primer binding and extension (Maizels & Weiner, 1994)+ The importance of the CCA end in translation and in replication suggests that its conformation will play a role in these two fundamental processes+ Previous studies indicate that the N73 nucleotide and aC 1 :A72 mismatch have the ability to manipulate the conformation of the CCA end+ An NMR analysis of tRNA acceptor stems (Puglisi et al+, 1994) showed that the CCA end preceded by an A73 nucleotide has an extended conformation, whereas that preceded by a U73 has a fold-back structure+ The fold-back structure of the CCA end brings the terminal A76 adjacent to the 59 side of the acceptor stem and may be stabilized by a weak hydrogen bond between A76 and U73+ The folded CCA end is also found in the crystal structure of the Escherichia coli initiator tRNA fMet which, despite having an A73, carries an unusual C1:A72 mismatch that is thought to contribute to the fold-back of the CCA end (Schevitz et al+, 1979; Woo et al+, 1980; Ferguson & Yang, 1986; Pscheidt & Wells, 1986)+ In tRNA fMet ,