Structural proteins associated with ribosomal cistrons in Xenopus laevis chromosomes

The N banding technique to define the location of nucleolus organiser in mammalian and marsupial chromosomes was applied to the Xenopus laevis chromosomes. Results obtained are: 1. The N bands coincide with the location of all the clustered ribosomal cistrons including the 18S + 28S RNA genes as well as the 5S RNA genes. 2. The N bands are consistently detected in both metabolically active (interphase) and metabolically inactive (metaphase) nuclei. 3. Cytochemical and chemical extraction tests indicate that the N bands show typical biochemical properties requested for non-histone (residual) chromosomal proteins. 4. Proteins associated with the 5S RNA genes differ, in their acid-solubility, from those for the 18S+28S RNA genes. 5. The N banding proteins comprise a small portion of a total nuclear protein. These findings strongly suggest the existence of ribosomal gene-specific non-histone proteins which probably represent the structural chromatin element rather than the primary gene product. The possible role of N banding proteins in eukaryotes is discussed.     

Transcriptional organization of the 5.8S ribosomal RNA cistron in Xenopus laevis ribosomal DNA.

Hybridization of purified, 32p-labeled 5.8S ribosomal RNA from Xenopus laevis to fragments generated from X. laevis rDNA by the restriction endonuclease, EcoRI, demonstrates that the 5.8S rRNA cistron lies within the transcribed region that links the 18S and 28S rRNA cistrons.