Biochemical Research on oogenesis. Composition of the 42-S storage particles of Xenopus laevix oocytes.

Previtellogenic oocytes of Xenopus 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 two prominent kinds of storage particles in previtellogenic oocytes of X. laevis. The smaller ones (7-S) contain about half 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 one molecule of 5-S RNA and one molecule of protein. In this paper we describe the biochemical and physical properties of the 42-S particles. The 42-S particle contains four main components: tRNA, 5-S RNA, a 50000-Mr protein (a) and a 40000-Mr protein (b) in the following molar ratios: 3/1/2/1. We propose a 28-component model for the 42-S particles. This model is consistent with all the biochemical and physical data that we report here. A 42-S particle is made up of four subunits, each of which contains three molecules of tRNA, one molecule of 5-S RNA, two molecules of protein a, and one molecule of protein b. Protein b from the 42-S particles and the 7-S particle protein are indistinguishable by all tests that we have tried. We present evidence showing that protein a binds tRNA whereas protein b binds 5-S RNA in the 42-S particles as well as in the 7-S particles.