Both the 5S rRNA gene and the AT-rich flanks of xenopus laevis oocyte-type 5S rDNA repeat are required for histone H1-dependent repression of transcription of pol III-type genes in in vitro reconstituted chromatin.

Incorporation of somatic histone H1 into chromatin during embryogenesis of Xenopus laevis results in repression of transcription of the oocyte- but not the somatic-type 5S rRNA genes. We showed earlier that a similar effect of the H1 observed in chromatin reconstituted on circular plasmids in vitro depends on its binding to the AT-rich flanks of the oocyte-type 5S rRNA gene. H1 binding results in stabilization of nucleosomes within the oocyte 5S rDNA repeat comprising the 5S rRNA gene with flanks and in reorganization of chromatin on the entire plasmid DNA. Performing in vitro transcription on reconstituted minichromosome templates carrying the oocyte 5S rRNA gene placed in different arrangements and at different distances from the AT-rich flanks, we now establish that the above effects of H1 observed upon its binding to the AT-rich sequences are absolutely dependent on the presence of the 120 bp oocyte 5S rRNA gene in its native position within the flanks. We also find that with the intact oocyte 5S rDNA repeat, the binding of H1 results in repression of transcription of both pol III- and pol II-type genes and that the transcriptionally inactive chromatin state spreads over a distance of at least a few nucleosomes.