Abstract
The formation of trifluoroleucine-tRNA and leucine-tRNA during embryonic development of the mouse is a function of gestational age. The existence of a leucine-tRNA synthetase has been demonstrated in early embryonic tissue (gestational age <10 days) which has unique chemical properties, acylating tRNA with trifluoroleucine with avidity almost equal to leucine. This enzyme has been shown to be stable to heating at 45 degrees C for 30 minutes.A second leucine-tRNA synthetase, which is similar chemically and physically to the enzyme isolated from more mature embryonic tissue, has been isolated from this embyronic source as well as from adult tissue. Quantitative changes in the species of tRNA isolated from these various sources have been documented utilizing the technique of polyacrylamide electrophoresis. These changes parallel the disappearance of the early embryonic enzyme, fewer species of tRNA acting as acceptors for leucine as maturation progresses.