Effects of ionic strength and state of assembly on kinetics of hydrogen exchange of calf thymus histones.

The kinetics of hydrogen exchange of calf thymus histone H2A-H2B dimers and (H3-H4)2 tetramers at pH 7 have been examined at low (0.16 M NaCl) and high (2 M NaCl) ionic strengths and after incorporation into (H2A-H2B-H3-H4)2 octamers. The similarity of the results for both species is noteworthy. Approximately 60% of the backbone amide protons are detectable in both low and high salt, and at least three kinetic phases can be distinguished. Increasing the ionic strength from 0.16 to 2 M accelerates exchange of some of the rapidly exchanging protons in both dimers and tetramers, while slowing exchange of others. Exchange of the more slowly exchanging protons is virtually unaffected. Incorporation of dimers into octamers accelerates exchange of approximately 40 protons to such an extent that they can no longer be detected. The effects of assembly upon the tetramer are qualitatively similar. These results indicate that both high ionic strengths and assembly destabilize some regions of the structure while stabilizing others. For both dimers and tetramers, the effects of ionic strength are dramatic, while those of assembly are more subtle. Higher resolution studies aimed at identifying the responsive protons would be of interest.