The DNase I sensitivity of Xenopus laevis genes transcribed by RNA polymerase III

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Abstract
Since the initial discovery that the DNase I sensitivity of the globin genes in different cell types correlates with globin gene expression1, this relationship has been shown to hold true for a variety of genes, including the genes for ovalbumin2, conalbumin3, α- and β-globin in chicken4,5, several heat-shock proteins in Drosophila6,7, the r-chromatin of Tetrahymena8 and the viral polyoma minichromosome9. Although genes transcribed by RNA polymerases I and II have been studied extensively, the genes transcribed by RNA polymerase III have not. We have therefore investigated the DNase I sensitivity of transfer RNA (tRNA) and oogenetic 5S RNA genes in the liver and erythocyte nuclei of Xenopus laevis. The oogenetic 5S genes are not transcribed in any known somatic cell10,11, and tRNA genes are transcribed in the hepatocyte but are inactive in the erythrocyte. We show here that, although in these two cell types the correspondence between DNase I sensitivity and gene transcription holds good for globin and the ribosomal genes, the tRNA and oogenetic 5S genes are DNase I sensitive in both liver and erythrocyte nuclei. Thus for the genes transcribed by polymerase III the correspondence of sensitivity and expression breaks down.