Differential nucleosome positioning on Xenopus oocyte and somatic 5 S RNA genes determines both TFIIIA and H1 binding: a mechanism for selective H1 repression.

In Xenopus somatic cells histone H1 effects the transcriptional repression of oocyte type 5 S RNA genes, without altering the transcription of the somatic type 5 S RNA genes. Using an unambiguous nucleosome mapping method we find substantial differences between the multiple in vitro nucleosome positions on the two types of genes. These nucleosome positions determine both transcription factor and H1 binding, allowing TFIIIA to bind more efficiently to nucleosomes containing the somatic 5 S RNA gene than to nucleosomes on the oocyte 5 S RNA gene. Significantly, in a binding competition between TFIIIA and H1, TFIIIA preferentially binds to the somatic nucleosome whereas H1 preferentially binds to the oocyte nucleosome, excluding TFIIIA binding. These results strongly suggest that nucleosome positioning plays a key role in the regulation of transcription of 5 S RNA genes and provide a molecular mechanism for the selective repression of the oocyte 5 S RNA genes by H1.