Association and dissociation of half molecules of phenylalanine specific tRNAs from yeast and wheat germ

From a number of tRNAs, half molecules have been prepared which by themselves are inactive but after recombination can be charged with amino acids in the in vitro acceptance assay [l-4] . Recently, active combinations were also obtained when half molecules from tRNAs of two different species, tRNAF&t and tRNAw$&t, were combined [5]. In the preceding paper the mechanism of the helixcoil transitions in half molecules of tRNAg& is described [6] . In the present paper, the results of thermodynamic and kinetic measurements on the half mole cules of tRNv& and tRNA${& and their homelogous and heterologous combinations are reported. Data on the stability of double helical structures at various temperatures in the presence and absence of magnesium are given. In contrast to the tRNAz& halves [6] the half molecules from tRNAt,lat possess base pairs additional to those normally written in cloverleaf models. This complicates the interpretation of the thermodynamic data on the recombination of the half molecules. The kinetic analysis, however, shows that in a heterologous combination the number of base pairs is lower than in the homologous corn binations. The observations of the preceding [6] and the present paper are discussed with regard to the acceptor activity for phenylalanine. 2. Materials and methods