The activation of amplified ribosomal RNA genes in the oocytes of Xenopus laevis: an electron microscope analysis.

Abstract

Mitchell, E. L. D. and Hill, R. S. 1987. The activation of amplified rihosomal genes in the oocytes of Xenopus laevis: an electron microscope analysis. —Hereditas 107 219–227. Lund, Sweden. ISSN 0018–0661. Received February 16, 1987 The Miller spreading technique has been used to study the ultrastructure of chromatin from the inactive, amplified ribosomal RNA genes found in the previtellogenic oocytes of Xenopus laevis and to see how this inactive structure is modified during the transcriptional activation of the genes in early vitellogenesis. Inactive, nucleolar chromatin fibres from 300 pm oocytes are packaged into nucleosomal and supranucleosomal structures of 20—-22 nm in diameter. However, where visible, the chromatin axis in active ribosomal genes has a smooth appearance and a diameter of 24 nm. By contrast, the axis of transcription units from lamphrush chromosomes, found in the same preparations, has a distinctively nucleosomal-type of organization. During transcriptional activation, in oocytes of 350–400 pm in diameter, the morphological appearance of the newly-activated genes varies considerably with respect to the length of the gene, the number of RNA polymerase molecules per gene and the amount of RNA transcript associated with the polymerases. However, in oocytes of at least 450 pm, the genes in most, but not all nucleoli, have the classical “Christmas Tree” appearance, consisting of densely packed RNA polymerases and transcripts which show a distinctive gradient in their length from the beginning to the end of the gene.