Higherorder Chromatin Structure

Histone Hi Binds to the Linker DNA between Nucleosomes

Once nucleosomes are formed, the next step in the packaging of DNA is the binding of histone HI. Like the core histones, Hi is a small, positively- charge d protein (Table 7-5). Hi interacts with the linker DNA between nucleosomes, further tightening the association of the DNA with tlie nudeosome, This can be detected by the increased protection of nucleusoma! DNA from micrococcal nuclease digestion. Thus, in contrast to the 147 bp protected by the core histones, addition of histone Hi to a nudeosome protects an additional 20 bp of DNA from micrococcal nuclease digestion.

Histone Hi has the unusual property of binding two distinct regions of the DNA duplex. Typically, these two regions are part of the same DNA molecule associated with a nudeosome (Figure 7-27). The sites of Hi binding are located asymmetrically relative Lo the nudeosome. One of the two regions bound by Hi is the linker DNA at one end of the nudeosome. The second site of DNA binding is in the middle of the associated 147 bp (the only DNA duplex present at the dyad axis). Thus, the additional DNA, protected from nuclease digestion described above, is restricted to linker DNA on only one side of the nudeosome. By bringing these two regions of DNA into close proximity, HI binding increases the length of the DNA wrapped lightly around the histone-octamer.

Hi binding produces a more defined angle of DNA entry and exit from the nudeosome (Figure 7-28). This effect, which can be visualized in the electron microscope, results in the nucleosomal DNA taking on a distinctly zigzag appearance. The angles of entry and exit vary substantially depending on conditions (including salt concentration, pH, and the presence of other proteins). If we assume these angles arc

FIGURE 7-28 The addition of Hi leads to more compact nudeosomal DNA. I he two images show an electron micrograph of nudeosomal DNA in the presence (a) and absence (b) of histone HI. Note the more compact and defined structure of the DNA in the presence of histone HI (Source Thorna et at involvement of histone HI in the organiia tion of the nudeosome. J Cell Biology, 83. 410, figs 4 8 6.)

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