Chromatin structure of the chicken adult beta-globin gene: is gene activity associated with local or long-range perturbations in chromatin structure?

We review our physical studies on the higher-order structure of bulk chromatin. Using the method of electric dichroism, we determine the arrangement of DNA in the 30-nm solenoid or thick chromatin fiber. With few assumptions, our results lead to a detailed model, in which the chromatosomes are arranged radially but tilted approximately 25 degrees from the solenoid axis. We then attempt to determine if the chromatin of the adult beta-globin gene in embryonic chicken red cells normally exists in a solenoidal configuration. We conclude that it does and that the stability of this globin gene-containing solenoid is similar to that of bulk DNA. Furthermore, the solenoid stability is not measurably perturbed by the binding of the nonhistone chromosomal proteins HMG 14 and 17. We describe the nature of a hypersensitive site close to the 5' end of the adult beta-globin gene. Approximately 200 bp of DNA appear accessible to nuclease digestion and moreover a substantial portion of this region can be excised as naked DNA. Overall there is strong evidence that gene activity is associated with local perturbations in chromatin structure but much weaker evidence that gene activity is associated with a perturbation in chromatin higher-order structure.