The Specific Template Activity of Chromatin

Summary When the chromatin isolated from duck reticulocytes is transcribed in vitro with E. coli RNA polymerase, the transcript is found to contain globin-specific RNA. In contrast, the transcript from duck erythrocyte nuclei contains much less globin RNA. We have made use of the tissue-specific template activity of reticulocyte chromatin to study the relationship between activity and the arrangement of proteins on the chromatin DNA. The enzyme staphylococcal nuclease is capable of digesting about half the DNA of reticulocyte chromatin, while the other half is protected from digestion by the bound proteins. We have isolated both the protected regions and the digestible regions of DNA. Using annealing methods, we show that the two kinds of DNA to a large extent have identical sequence populations, arising from an initial random distribution of most of the proteins on DNA binding sites. We have also studied the annealing of the protected and digestible regions to globin-specific DNA. In this case the regions have distinctly different annealing characteristics, reflecting a non-random distribution of proteins on the globin gene. This distribution appears to be correlated with the transcriptional activity. The results indicate that an actively transcribed globin gene is at least partly covered by protein. We have also examined the reassociation of reticulocyte chromatin proteins with protein-free DNA, and find that the reconstituted complex regains a large part of the specific transcriptional activity present in the intact chromatin. The ease of specific reassociation may be related to the relatively non-specific arrangement of most chromatin proteins on DNA binding sites.