Transcription of Xenopus borealis rRNA genes in nuclei of Xenopus laevis oocytes

Abstract

Recombinant plasmids that contained a single rRNA gene and spacer from Xenopus borealis were injected into the nuclei of Xenopus laevis oocytes. After spreading the contents of injected nuclei for electron microscopy, we found transcription units indicative of the accurate transcription of the borealis gene. Both the number of RNA polymerase molecules per transcription unit and the total number of active genes per nucleus were similar to those found for injected laevis genes. We also injected laevis oocytes with a plasmid that contained both a shortened laevis rRNA gene and a full-length borealis rRNA gene. Visualization of transcription on this plasmid showed that usually either a short or a long transcription unit was present on any one molecule. The morphologies of the short and long transcription units were consistent with their representing transcription of the laevis and borealis genes, respectively. Both types of transcription unit were present in roughly equal numbers. Since the laevis oocyte transcribed injected borealis rRNA genes as efficiently as it transcribed injected laevis rRNA genes, it stands in marked contrast to the situation in embryos of interspecific hybrids between laevis and borealis where transcription occurs only from the laevis rRNA genes. Possible explanations for the difference in transcriptional behavior of the borealis rRNA gene in hybrid embryos and laevis oocytes are discussed.