Structure and expression of the genes encoding the alpha and beta subunits of yeast phenylalanyl-tRNA synthetase.

Abstract

The two genes FRS1 and FRS2 encoding, respectively, the large (alpha) and small (beta) subunits of cytoplasmic phenylalanyl-tRNA synthetase from bakers' yeast have been cloned and sequenced. The derived protein primary structures are confirmed by peptide sequences evenly distributed along the reading frames. These predict a subunit Mr of 67,347 for alpha and 57,433 for beta, in good agreement with earlier determinations carried out on the purified protein. These subunit sequences have been compared to those of Escherichia coli phenylalanyl-tRNA synthetase as well as to the small beta subunit of the corresponding yeast mitochondrial enzyme; limited but significant homology was found between the two alpha subunits on the one hand and between the three beta subunits on the other hand. The results suggest that these three enzymes, from E. coli, yeast cytoplasm, and yeast mitochondria, have strongly diverged from one another. The initiation sites of transcription have been determined for both yeast genes. Their 5'-upstream regions show no sequence similarities that would have indicated a coordinate control of gene expression at the transcriptional level. Measurements of steady-state levels of FRS-mRNAs in overproducing strains indicate that there is no restriction in mRNA synthesis. Therefore the control of gene expression, leading to a balanced synthesis of alpha and beta subunits, is likely to occur at the translational level.