| Abstract: | Yeast tRNA-Phe has been cross-linked photochemically to three aminoacyl-tRNA synthetases, yeast phenylalanyl-tRNA synthetase, Escherichia coli isoleucyl-tRNA synthetase, and E. coli valyl-tRNA synthetase. The two non-cognate enzymes are known to interact with tRNA-Phe. In each complex, three regions on the tRNA are found to cross-link. Two of these are common to all of the complexes, while the third is unique to each. Thus, the cognate and non-cognate complexes bear considerable similarity to each other in the way in which the respective enzyme orients on tRNA-Phe, a result which was also established for the complexes of E. coli tRNA-Ile (BUDZIK, G.P., LAM, S.M., SCHOEMAKER, H.J.P., and SCHIMMEL, P.R. (1975) J. Biol. Chem. 250, 4433-4439). The common regions include a piece extending from the 5'-side of the acceptor stem to the beginning of the dihydrouridine helix, and a segment running from the 3' side of the extra loop into the TpsiC helix. These two regions overlap with and include some of the homologous bases found in eight tRNAs aminoacylated by yeast phenylalanyl-tRNA synthetase (ROE, B., SIROVER, M., and DUDOCK, B. (1973) Biochemistry 12, 4146-4153). Although well separated in the primary and secondary structure, these two segments are in close proximity in the crystallographic tertiary structure. In two of the complexes, the third cross-linked fragment is near to the two common ones. The picture which emerges is that the enzymes all interact with the general area in which the two helical branches of the L-shaped tertiary structure fuse together, with additional interactions on other parts of the tRNAas well. |
