DIFFERENTIAL NUCLEOSOME POSITIONING ON XENOPUS OOCYTE AND SOMATIC 5-SRNA GENES DETERMINES BOTH TFIIIA AND H1 BINDING - A MECHANISM FOR SELECTIVE H1 REPRESSION

In Xenopus somatic cells histone H1 effects the transcriptional repres sion of oocyte type 5 S RNA genes, without altering the transcription of the somatic type 5 S RNA genes. Using an unambiguous nucleosome map ping method we find substantial differences between the multiple in vi tro nucleosome positions on the two types of genes. These nucleosome p ositions 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. Significa ntly, in a binding competition between TFIIIA and H1, TFIIIA preferent ially binds to the somatic nucleosome whereas H1 preferentially binds to the oocyte nucleosome, excluding TFIIIA binding. These results stro ngly suggest that nucleosome positioning plays a key role in the regul ation of transcription of 5 S RNA genes and provide a molecular mechan ism for the selective repression of the oocyte 5 S RNA genes by H1. (C ) 1998 Academic Press.