Exchange of histones H1 and H5 between chromatin fragments. A preference of H5 for higher-order structures.

Histones H1 and H5 can exchange between an H1-containing chromatin fragment from rat liver and an H1, H5-containing fragment from chicken erythrocytes at ionic strengths from about 35 mM to 105 mM. The redistribution has reached equilibrium by ionic strength 75 mM in 1 h or less at 4 degrees C. After exchange at ionic strength 75 mM, long fragments, whether of rat liver or chicken erythrocyte chromatin, are recovered with a higher H5:H1 ratio than short fragments, suggesting a stronger preference of H5 than of H1 for higher-order structures which exist for long fragments at ionic strength 75 mM. Competition experiments between occupied H1 or H5 binding sites on chromatin fragments from rat liver or chicken erythrocytes and empty sites on H1-depleted rat chromatin show that rat H1 does not distinguish between the two types of site, whereas H5 discriminates in favour of sites on native chromatin, even when the chicken fragments are too short to form higher-order structures. (The behavior of the chicken H1, which may be bound less tightly than rat H1, depends on the length of the chicken fragment, in a manner suggesting that fragments of 15 nucleosomes and longer can form stable higher-order structures which have high-affinity binding sites for both H5 and H1.) We conclude that the affinity of sites for H5 is in the order:higher-order structures greater than nucleosome filament much greater than H1-depleted chromatin. The same relative order of affinities may well apply for H1 but the discrimination is much lower. This difference between H1 and H5 seems likely to be relevant to the greater stability of H5-containing chromatin, and in turn its transcriptional inactivity, and indeed to the mechanism of replacement of H1 by H5 during the terminal stages of erythropoiesis.