Stability of the higher-order structure of chicken-erythrocyte chromatin in solution.

Abstract

A detailed hydrodynamic study of chicken erythrocyte chromatin as a function of ionic strength, and comparison with results reported earlier for rat liver chromatin [Butler, P.J.G. and Thomas, J. O. (1980) J. Mol. Biol. 140, 505–529; Thomas, J. O. and Butler, P.J.G. (1980) J. Mol. Biol. 144, 89-93] shows similar behaviour in the two cases, but quantitative differences indicate a greater stability of the chicken erythrocyte chromatin. The hydrodynamic method may thus provide a general assay for the stability in solution of the higher-order structure of chromatin–a helical coiling into a solenoid. When sedimentation coefficients of nucleosome oligomers of various lengths from chicken erythrocytes are measured as a function of ionic strength, two main changes in behaviour are observed. The first is between pentamer and hexamer as in rat liver chromatin, suggesting that the solenoid has 5–6 nucleosomes per turn. The longer repeat length of chicken erythrocyte chromatin (212 base pairs compared with 200 base pairs for rat liver chromatin) is thus accommodated into the solenoid without obvious change of structure. The second change in behaviour is a jump of about 9% in the sedimentation coefficient at ionic strength 50 mM for nucleosome oligomers above a critical size, which we attribute to the relative instability of higher order structure in long oligomers at lower ionic strength. In chicken erythrocyte chromatin this critical size is about 60 nucleosomes, compared with 50 nucleosomes in rat liver chromatin. The increased axial stability in the solenoid that this indicates possibly derives from the erythrocyte-specific histone H5, and would be expected in this chromatin which is virtually inactive in transcription.