Transient activation of oocyte 5S RNA genes in Xenopus embryos by raising the level of the trans-acting factor TFIIIA.

The concentration of the trans-acting factor, TFIIIA, required for the activation of 5S RNA genes in Xenopus can be elevated in developing embryos by injecting a synthetic full-length mRNA into fertilized eggs. 5S RNA genes are activated by the increased factor concentration at the mid-blastula transition through mid-gastrulation. The activated oocyte 5S RNA genes are then inactivated, leaving the TFIIIA-enhanced embryos with the same profile of differential 5S RNA gene activity as control embryos, i.e., synthesizing mainly somatic 5S RNA. Inactivation of the oocyte 5S RNA genes is complete by neurulation and can occur in the absence of DNA replication. We propose that this loss of gene activity is due at least in part to destabilization of transcription complexes that are associated with oocyte 5S RNA genes.