The rapid transfer and selective association of histones H2A and H2B onto negatively coiled DNA at physiological ionic strength.

The interaction of histones H2A and H2B with closed circular DNA maintained in either a positive or negatively coiled state has been studied. The methods involved separation of complexes by sucrose gradients and by agarose gel electrophoresis. Histones H2A and H2B have a 10-fold preference for DNA that contains performed negative coils as compared to DNA that is positively coiled or topologically unrestrained. It was also observed that histones H2A and H2B, when initially reconstituted onto positively coiled DNA, transfer very efficiently to negatively coiled DNA at 100 mM NaCl. Transfer is virtually complete within 5 s at 4 degrees C. By utilizing a cross-linking agent (disuccinimidyl suberate), it was found that histones H2A and H2B could be cross-linked into a tetrameric structure when on negatively coiled DNA but not on positively coiled DNA. It is concluded that protein-protein interactions facilitate the transfer. The importance of these protein-protein interactions is further demonstrated in experiments in which H3,H4 were included, and, in this instance, the strong interaction of H2A,H2B with H3,H4 negates the preference of H2A,H2B for negatively coiled DNA. These observations are discussed with regard to the mobility of histones during the transcription process during which transcription-induced stresses may be produced.