THE ANTICODON LOOP IS A MAJOR IDENTITY DETERMINANT OF SACCHAROMYCES-CEREVISIAE TRNA(LEU)

The recognition of tRNA(Leu), one of the class II tRNAs having a long variable arm, by leucyl-tRNA synthetase in Saccharomyces cerevisiae wa s studied using the T7 transcription system. Exchanging the anticodon arm of tRNA(Leu) but not the D- or T Psi C-arm to that of tRNA(Ser) se riously affected the leucine accepting activity. Two nucleotides in th e anticodon loop, A35 and G37, were found to be important for leucylat ion. It was also found that the discriminator base, A73, is required f or leucylation, and G73 of tRNA(Ser) functions as a negative identity determinant for leucyl-tRNA synthetase. Introducing a set of three bas e substitutions at positions 35, 37 and 73 was sufficient to convert t RNA(Ser) into an efficient leucine acceptor. These results indicate th at the identity elements of tRNA(Leu) lie at the second position of th e anticodon and the 3' adjacent to the anticodon as well as the discri minator position. Such a sequence specific recognition manner is signi ficantly different from that of Escherichia coli, in which not the ant icodon but the tertiary structural elements play a key role in discrim inating from other class II tRNAs. The leucine system is the first exa mple which shows that the requirement of the anticodon sequence is var iable among species. (C) 1996 Academic Press Limited