A mosaicism in the higher order structure of Xenopus oocyte nucleolar chromatin prior to and during ribosomal gene transcription.

We report an analysis by electron microscopy of the differences in the folding of ribosomal gene and adjacent nontranscribed spacer DNA of Xenopus laevis oocytes into supranucleosomal chromatin structures. The chromatin structures identified in gene and spacer regions of transcriptionally active nucleoli (from stage 5 oocytes) were compared with those found in nucleoli prior to transcription (from stage 2 oocytes) to determine whether changes in the chromatin structure occur when transcription is initiated. Chromatin structures were characterized by their morphology and by the extent of folding of DNA in chromatin. Nontranscribed spacer regions from both transcriptionally active and inactive nucleoli appear to be packaged into supranucleosomal structures and are contracted by a factor of at least 20 from the length of B form DNA. The structure of the adjacent gene region, both before and during transcription, is much more extended; the only structures observed are the size of nucleosomes, and the DNA is contracted by a factor of 1.4 from its B form length. Thus a mosaicism in the higher order structure of gene and spacer rDNA is established days or weeks prior to the initiation of transcription of these genes and maintained during transcriptionally active stages.