| Abstract: | Rat liver chromatin core particles digested with clostripain yield a structurally well-defined nucleoprotein particle with an octameric core made up of fragmented histone species (designated H'2A, H'2B, H'3 and H'4, respectively) after selective loss of a sequence segment located in the N-terminal region of each core histone. Sequential Edman degradation and carboxypeptidase digestion unambiguously establish that histones H2A, H2B, H3 and H4 are selectively cleaved at the carboxyl side of Arg 11, Lys 20, Arg 26 and Arg 19 respectively and that the C-terminal sequences remain unaffected. Despite the loss of the highly basic N-terminal regions, including approximately 17% of the total amino acids, the characteristic structural organization of the nucleosome core particle appears to be fully retained in the proteolyzed core particle, as judged by physicochemical and biochemical evidence. Binding of spermidine to native and proteolyzed core particles shows that DNA accessibility differs markedly in both structures. As expected the proteolyzed particle, which has lost all the in vivo acetylation sites, is not enzymatically acetylated, in contrast to the native particle. However, proteolyzed histones act as substrates of the acetyltransferase in the absence of DNA, as a consequence of the occurrence of potential acetylation sites in the core histones thus rendered accessible. The possible role of the histone N-terminal regions on chromatin structure and function is discussed in the light of the present observations with the new core particle obtained by clostripain proteolysis. |
