Recherches biochimiques sur l'oogenése

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Abstract
As found previously, 70 to 75% of the RNA present in previtellogenic oocytes of Xenopus laevis (average diameter: 50 to 150 μm) is 5‐S RNA and transfer RNA. In this paper, we show that most of the RNA of the small oocytes is stored in nucleoprotein particles sedimenting at 58 S and 42 S. These particles are no longer detectable in vitellogenic oocytes (average diameter: 375 to 1100 μm) which accumulate ribosomes very rapidly.The 58‐S particles contain mainly high‐molecular‐weight RNA. This RNA has a base composition close to that of DNA. Its main component sediments at about 19 S. The 42‐S particles contain more than half of the RNA and a quarter of the protein present in homogenates of small oocytes. The only RNA species found in the 42‐S particles are 5‐S RNA and tRNA. The molar ratio of tRNA to 5‐S RNA in the 42‐S particles is 3:1.5‐S RNA is distributed as follows in the small oocytes: less than 1% of the total cell content is bound to the ribosomes; about 50% is in the 42‐S particles and the remainder is apparently free in the cell sap. By its base composition and its hybridization properties 5‐S RNA from the 42‐S particles cannot be distinguished from 5‐S RNA of the cell sap and from 5‐S RNA of somatic cells.About 90% of the total tRNA content of the oocyte is included in the 42‐S particles. The remaining 10% is either free or bound to the ribosomes. No difference was noticed in the hybridization properties of tRNA which is inside the 42‐S particles and tRNA which is outside these particles.The 42‐S particles present in homogenates of previtellogenic oocytes are not artifacts. These particles are probably storage particles in which 5‐S RNA and tRNA produced in excess during the initial period of oogenesis are kept for many months until the ribosomes begin to accumulate. The role of the 58‐S particles remains unknown.