Codon-induced Structural Transitions in tRNA

With the structure of an RNA known in one conformation, one may ask whether this RNA molecule may change its conformation during its biological function (Kurland et al. 1975). Such could occur during aminoacylation (Dvorak et al. 1976Dvorak et al. 1978), in the formation of the ternary complex aminoacyl-tRNA · EF-Tu · GTP (Pingoud et al. 1978), or during the codon-directed binding of aminoacyl-tRNA to the ribosome (Schwarz et al. 1976; Schwarz and Gassen 1977). (EF-Tu is elongation factor Tu.) In this paper we restrict ourselves to structural transitions that occur during the codon-directed binding of either initiator or elongator tRNA to the ribosome. Conformational changes in the anticodon-loop region of the tRNA were postulated by Woese (1970). In an extension of the Fuller and Hodgson (1967) model, he proposed a transition between the 3′-stacked and the 5′-stacked conformation as a basic mechanism for mRNA translocation. Recently, Urbanke and Maass (1978) measured the temperature dependence of the fluorescence of the “Y-base” in yeast tRNA phe . A slow structural transition characterized by a monomolecular all-or-none effect was found. These authors, too, discuss as a possible explanation for their data a transition from the 3′-stacked (the more stable one) to the 5′-stacked anticodon structure (Urbanke and Maass 1978). Similar experiments were performed earlier by Yoon et al. (1975), who arrived at similar conclusions. Although C32 and U33 are not available for binding in the crystal lattice of tRNA phe , UUCA and the pentamer UUCAG are bound more effectively to the anticodon region of the tRNA than...