Biochemical research on oogenesis. Transfer RNA is fully charged in the 42-S storage particles of Xenopus laevis oocytes.

1. Transfer RNA makes up 30-40% of total RNA in previtellogenic oocytes of Xenopus laevis. The bulk of tRNA is associated with 5-S RNA and two proteins in a high-molecular-weight complex sedimenting at 42S. 2. We show here that all kinds of tRNA are present in the 42-S particles and all of them sediment coincidently. Particle tRNA is fully charged in vivo. During purification of the 42-S particles tRNA becomes partially uncharged. When purified particles are incubated in vitro with amino acids and ATP a charging reaction occurs without disruption of the nucleoprotein complex. Many aminoacyl-tRNA synthetases can be shown to co-sediment with the 42-S particles. We conclude that complete aminoacylation of tRNA within the storage particles results from the activity of particle-bound aminoacyl-tRNA synthetases.