Biochemical Research on Oogenesis. Composition of the 42-S Storage Particles of Xenopus laevis Oocytes

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Abstract
Previtellogenic oocytes of X. laevis contain far more 5-S RNA and tRNA than 28-S + 18-S RNA. tRNA and 5-S RNA are storage products that will be used in later oogenesis for protein synthesis and ribosome assembly. Stored tRNA and 5-S RNA are not free in the cell sap but belong to nucleoprotein particles of various sizes. There are 2 prominent kinds of storage particles in previtellogenic oocytes of X. laevis. The smaller ones (7-S) contain .apprx. 1/2 of the cell''s 5-S RNA. The larger ones (42-S) contain the remainder of the 5-S RNA and 90% of the tRNA. The 7-S particles consist of 1 molecule of 5-S RNA and 1 molecule of protein. The biochemical and physical properties of the 42-S particles are described. The 42-S particle contains 4 main components: tRNA, 5-S RNA, a 50,000 MW protein and a 40,000 MW protein in the following molar ratios: 3/1/2/1. A 28 component model is proposed for the 42-S particles. This model is consistent with all the biochemical and physical data that are reported. A 42-S particle is made up of 4 subunits, each of which contains 3 molecules of tRNA, 1 molecule of 5-S RNA, 2 molecules of protein a and 1 molecule of protein b. Protein b from the 42-S particles and the 7-S particle protein are indistinguishable by all tests that were tried. Protein a binds tRNA whereas protein b binds 5-S RNA in the 42-S and in the 7-S particles.