SynopsisNative, NaC1-treated, trypsin-treated, and polylysine-bound nucleohistones were studied in 2.5 X M EDTA, pH 8.0, using circular dichroism (CD) and thermal denaturation. Removal of histone I by 0.6 M NaCl has a much smaller effect on both A E Z Z~and Ae~,s than the removal of other histones. This indicates that histone I has less helical content and less conformational effect on the DNA in nucleohistone. By extrapolating to 10007, binding by histones other than I, the positive CD band near 275 nm is close to zero. Comparison is also made between the effects of binding by the more basic and the less basic halves of histones by trypsin-digestion and polylysinebinding experiments. Trypsin digestion of nucleohistone reduces melting band IV at 82°C much more than melting band I11 a t 72°C. However, the CD changes of Aezn and At220 induced by trypsin digestion are small, unless melting band I11 is also reduced by the use of a higher trypsin level. This implies that the less basic halves of histones, which stabilize DNA to 72°C (melting band 111), have more helical structure and are more responsible for conformational change in DNA than are the more basic halves, which stabilize DNA to 82°C (melting band IV). Polylysine binding to nucleohistone diminishes melting band I11 but has no effect on melting band IV. This binding affects only slightly the At220 of nucleohistone, indicating that polylysine interferes very little with the structure of the less basic halves of bound histones. The implications of these studies with respect to chromatin structure are discussed.