The distribution of oocyte 5S, somatic 5S and 18S + 28S rDNA sequences in the lampbrush chromosomes of Xenopus laevis

The nucleolus organizer locus of Xenopus laevis lampbrush chromosomes was identified by in situ hybridization of a super(3)H-labelled probe complementary to 18S + 28S rDNA. The nucleolus organizer is an axial granule on chromosome III that lies four-fifths the way down this chromosome reading from its larger (left) telomere, just within an "exploded" region that extends to its right end, where the lateral loops are exceptionally long. The multiple sites of oocyte and somatic 5S gene families were identified. The authors suggest that 5S sequences on resolvable loops are transcribed by readthrough from upstream promoters, probably by polymerase II.