A SINGLE HIGH-AFFINITY BINDING-SITE FOR HISTONE H1 IN A NUCLEOSOME CONTAINING THE XENOPUS-BOREALIS 5-S RIBOSOMAL-RNA GENE

We have reconstituted nucleosomes containing the Xenopus borealis 5 S rRNA gene, a single histone octamer, and 1 or 2 molecules of histone H 1. We determine that the 1st molecule of histone H1 to associate with the 5 S nucleosome binds with high affinity (K-D similar to 2 nM), and the 2nd molecule of H1 binds with a reduced affinity (K-D similar to 10 nM). This latter binding is comparable with the association of hist one H1 with naked DNA. Neither molecule of histone al alters the helic al periodicity of DNA in the nucleosome as revealed by hydroxyl radica l cleavage. We conclude that although multiple molecules of histone H1 can associate with nucleosomal DNA, there is only a single high affin ity binding site for histone H1 within the 5 S nucleosome.