Onset of 5 S RNA gene regulation during Xenopus embryogenesis.

The transcription of 5 S RNA genes during oogenesis results in the storage of sufficient 5 S RNA in ribosomes to support subsequent embryogenesis. Xenopus oocytes of all stages synthesize oocyte-type 5 S RNA. A generalized repression of transcription occurs at meiosis and is maintained throughout early cleavage. The onset of 5 S RNA synthesis is detected at approximately the 4000-cell blastula stage (stage 9), concomitant with de novo synthesis of other species of RNA. At this developmental stage the level of 5 S RNA synthesis is low relative to the synthesis of tRNA and small nuclear RNAs. Analysis of this newly synthesized 5 S RNA reveals it to be a nearly equal mixture of oocyte and somatic 5 S RNA derived from both maternal and paternal genes. Given the 50:1 ratio of oocyte to somatic 5 S RNA genes in X. laevis, these results indicate that the majority of the oocyte 5 S RNA genes are inactivated at this time. This reflects differential transcription of the two families of 5 S RNA genes rather than post-transcriptional stability as demonstrated by the ability of a chromatin template isolated from stage 9 embryos to direct the same ratio of oocyte to somatic 5 S RNA synthesis in vitro as that observed in vivo. By completion of gastrulation, 5 S RNA synthesized in vivo and directed from chromatin in vitro is at least 90% somatic 5 S RNA. These results are consistent with a model in which the decrease in concentration of the 5 S-specific transcription factor relative to the number of 5 S RNA genes during embryogenesis contributes to the inactivation of the oocyte 5 S RNA genes.