Interaction of HMG 14 and 17 with actively transcribed genes.

The interaction of HMG 14 and 17 with actively transcribed genes was studied by monitoring the sensitivity of specific genes to DNAase I after reconstitution of HMG-depleted chromatin with HMG 14 and 17. Our experiments lead to the following conclusions: most actively transcribed genes become sensitized to DNAase I by HMG 14 and 17; either HMG 14 or HMG 17 can sensitize most genes to DNAase I; genes transcribed at different rates have about the same affinity for HMG 14 and 17; HMG 14 and 17 bind stoichiometrically to actively transcribed nucleosomes; and HMG 14 and 17 can restore DNAase I sensitivity to purified nucleosome core particles depleted to HMGs. This last observation suggests that during reconstitution, low levels of HMG 14 and 17 can associate with the active nucleosomes in the presence of a 10--20 fold excess of inactive nucleosomes. Consequently, we conclude that besides their association with HMGs, active nucleosomes also have at least one other unique feature that distinguishes them from bulk nucleosomes and insures proper HMG binding during reconstitution.