THE C-TERMINAL EXTENSION OF YEAST SERYL-TRANSFER-RNA SYNTHETASE AFFECTS STABILITY OF THE ENZYME AND ITS SUBSTRATE AFFINITY

Saccharomyces cerevisiae seryl-tRNA synthetase (SerRS) contains a 20-a mino acid C-terminal extension, which is not found in prokaryotic SerR S enzymes. A truncated yeast SES1 gene, lacking the 60 base pairs that encode this C-terminal domain, is able to complement a yeast SES1 nul l allele strain; thus, the C-terminal extension in SerRS is dispensabl e for the viability of the cell. However, the removal of the C-termina l peptide affects both stability of the enzyme and its affinity for th e substrates. The truncation mutant binds tRNA with 3.6-fold higher af finity, while the K-m for serine is 4-fold increased relative to the w ild-type SerRS. This indicates the importance of the C-terminal extens ion in maintaining the overall structure of SerRS.