Instability of the nuclear chromatin of procyclic Trypanosoma brucei brucei

Digestion of nuclear chromatin of Trypanosoma brucei brucei procyclic culture forms with micrococcal nuclease yielded DNA fragments which formed DNA ladders in agarose gels, similar to those of rat liver. However, the chromatin of trypanosomes was digested more rapidly. The digestion of T. b. brucei chromatin yielded a large amount of DNA fragments of core-particle size. The numbers of base pairs per nucleosomal and linker DNA were identical in both species, if the digestion conditions were reduced in the case of T. b. brucei. Psoralen cross-linking of soluble chromatin of trypanosomes at 5 mM salt at pH 7 or pH 10 resulted in an irregular array of single-stranded (ss) bubbles separated by variable stretches of double-stranded (ds) DNA. The proportion of ss DNA was low compared with the ratio of ss/ds stretches in rat liver chromatin, which also showed regularly arranged nucleosomal DNA. Soluble chromatin of T. b. brucei, pre-treated with 500 mM NaCl to remove a potential H1 and psoralen cross-linked at 5 mM salt at pH 7 or pH 10 was to a great extent ds in both situations. The true nucleosome filament organization of T. b. brucei chromatin could only be shown by psoralen cross-linking the DNA in whole nuclei under physiological conditions. The results indicate that the chromatin of procyclic T. b. brucei differs significantly in its compaction pattern from rat liver chromatin; a typical histone H1 is not found, and the DNA-protein interactions are also less stable and can more easily be destabilized by experimental conditions.