Distribution of tissue‐specific tightly bound non‐histone proteins in the first level of repeating chromatin structures

Non‐histone proteins, tightly bound to DNA, have been extracted from whole chromatin and core particles prepared from pig liver or kidney. We have investigated by bidimensional slab gel electrophoresis the distribution of this protein class in the first level of repeating structure of chromatin. Our results reveal that non‐histone proteins tightly bound to DNA are a heterogeneous protein class. Some of them, particularly in the core particles, appear to be essentially the same in both tissues, though having differences in their isoelectric point, which may be attributed to postsynthetic modifications. We have calculated that this protein class is associated to only 10% of nucleosomes, these nucleosomes having, on the average, one protein molecule for each core DNA. The tissue‐specific proteins have high molecular mass (ranging from 135 kDa to 70 kDa in liver, over 135 kDa in kidney) and, in kidney, a more basic isoelectric point. These proteins are mainly located outside the core particles; they could be situated in the spacer regions and/or be involved in determining higher levels of chromatin organization.