Valyl-tRNA and leucyl-tRNA synthetases in wheat germ and seedlings

Valyl- and leucyl-tRNA synthetases (Val-RS and Leu-RS) isolated from wheat germ and seedlings were separated by chromatography on hydroxylapatite into organellar (Val-RS I and Leu-RS I) and cytoplasmic (Val-RS II and Leu-RS II) enzymes; the enzyme extracted from isolated chloroplasts and mitochondria corresponded to the RS I fractions. It was proved by RPC-5 chromatography of tRNA Val that Val-RS I and Val-RS II recognized all five isoacceptor tRNA Val species both from wheat germ and leaves, as well as tRNA Val from E. coli. However, out of the six isoacceptor tRNA Leu species, Leu-RS II aminoacylated two cytoplasmic species only, while Leu-RS I, the remaining four organellar tRNA Leu fractions. Both leucyl-tRNA synthetases charged E. coli tRNA, Leu-RS I more effectively than Leu-RS II. The absence of fraction RS I in cytosol seems to indicate that valyl- and leucyl-tRNA synthetases (coded for by the nuclear genome) were modified to the organellar forms after (or during) passage into organelles.