Histone-DNA interactions in the chromatin of procyclic Trypanosoma brucei brucei.

The dissociation of histone proteins a-d from the chromatin of Trypanosoma brucei brucei procyclic culture forms was investigated by removing the proteins from the DNA by centrifugation of soluble chromatin through isokinetic sucrose gradients in the presence of NaCl. The dissociation of the T. b. brucei histones was compared with that of their higher-eukaryote counterparts H3, H2A, H2B and H4. All four histones of T. b. brucei remained bound to the DNA at 500 mM NaCl, were partially released at 750 mM NaCl and were completely dissociated from the DNA at 1 M NaCl. These interactions of histones a-d with the DNA were comparable with those of the H2 histones in the chromatin of higher eukaryotes, and histones a and d interacted with the DNA more weakly than did their higher-eukaryote counterparts H3 and H4. Substoichiometric amounts of an additional protein were recovered in the top fractions of the gradients under all dissociation conditions. This protein migrated in the H1 region of rat-liver chromatin in various gel systems. Its early release from the DNA also indicated a resemblance to histone H1. The presence of only small amounts of this protein and the relatively weak interactions of histones a and d with the DNA suggest that the mechanisms involved in chromatin compaction in T. b. brucei are different from those in higher eukaryotes.