The structure of the globin genes in chromatin.

The distribution of proteins in the neighborhood of the globin genes of duck reticulocyte chromatin has been studied. This chromatin is first shown to be an active template for transcription in vitro of globin messenger-like RNA. The chromatin is then treated with staphylococcal nuclease and the DNA fragments protected from nuclease attack ("covered DNA") are isolated. Alternatively, the chromatin is titrated with poly-D-lysine, and by successive treatment with Pronase and nuclease, the DNA regions accessible to polylysine are isolated (open "DNA"). In order to determine the distribution of globin gene sequences in open and covered DNA, these two fractions are annealed to globin cDNA (globin probe). It is found that while all globin gene sequences are represented in covered DNA, a specific portion of the globin gene is missing from open DNA, corresponding to about 20% of the gene length. It is concluded that specific regions of the globin genes of reticulocyte chromatin are partly covered by proteins in such a way as to render them in accessible to polylysine. In contrast, no difference is observed in the annealing properties of open and covered regions to globin probe using DNA isolated from erythrocyte chromatin, which is a poor templete in vitro for production of globin message. The annealing of open and covered DNA to each other has also been studied. It is found that open and covered DNA have identical sequence populations. Thus, in contrast to the special arrangement of proteins in the neighborhood of the globin gene, there does not appear to be any sequence-specific arrangement of the bulk of the chromatin proteins on chromatin DNA.