This invention relates to isolated nucleic acid fragments encoding aminoacyl-tRNA synthetases and particulary lsoleucyl-, lysyl-, phenylalanyl- and prolyl-tRNA synthetases from Zea mays, Oryza sativa, Glycine max and Tritium aestivum. The invention also relates to the construction of a chimeric gene encoding all or a portion of the aminoacyl-tRNA synthetase, in sense or antisense orientation, wherein expression of the chimeric gene results in production of altered levels of the aminoacyl-tRNA synthetase in a transformed host cell.