Auld D S, Schimmel P
Department of Biology, Massachusetts Institute of Technology, Cambridge 02139, USA.
Science. 1995 Mar 31;267(5206):1994-6. doi: 10.1126/science.7701322.
The genetic code is based on specific interactions between transfer RNA (tRNA) synthetases and their cognate tRNAs. The anticodons for methionine and isoleucine tRNAs differ by a single nucleotide, and changing this nucleotide in an isoleucine tRNA is sufficient to change aminoacylation specificity to methionine. Results of combinatorial mutagenesis of an anticodon-binding-helix loop peptide were used to design a hybrid sequence composed of amino acid residues from methionyl- and isoleucyl-tRNA synthetases. When the hybrid sequence was transplanted into isoleucyl-tRNA synthetase, active enzyme was generated in vivo and in vitro. The transplanted peptide did not confer function to methionyl-tRNA synthetase, but the substitution of a single amino acid within the transplanted peptide conferred methionylation and prevented isoleucylation. Thus, the swap of a single amino acid in the transplanted peptide switches specificity between anticodons that differ by one nucleotide.
遗传密码基于转运RNA（tRNA）合成酶与其同源tRNA之间的特定相互作用。甲硫氨酸和异亮氨酸tRNA的反密码子仅相差一个核苷酸，在异亮氨酸tRNA中改变这个核苷酸足以将氨酰化特异性改变为甲硫氨酸。反密码子结合螺旋环肽的组合诱变结果被用于设计一种由甲硫氨酰-tRNA合成酶和异亮氨酰-tRNA合成酶的氨基酸残基组成的杂合序列。当将该杂合序列移植到异亮氨酰-tRNA合成酶中时，在体内和体外均产生了活性酶。移植的肽并未赋予甲硫氨酰-tRNA合成酶功能，但移植肽内单个氨基酸的替换赋予了甲硫氨酰化功能并阻止了异亮氨酰化。因此，移植肽中单个氨基酸的交换在相差一个核苷酸的反密码子之间切换特异性。
