Abstract
To elucidate the molecular features of active chromatin, we have mapped, by two-dimensional electrophoresis, the protein composition of nucleosomes that package the immunoglobulin kappa chain gene of mouse plasmacytoma cells. Nucleoprotein particles that possess the active kappa chain gene comigrate with bulk mononucleosomes that contain high mobility group proteins HMG-14 or -17 but lack histone H1. High electrophoretic resolution of the underlying core particles, after removal of ubiquitin by isopeptidase treatment, reveals that these nucleosomes are nonubiquitinated, even though they coincidently migrate with bulk ubiquitinated particles. This distinctive electrophoretic behavior may be correlated with the presence of histone H2A.X. Nucleosomes exhibiting these unusual properties appear to span at least 10 kilobases, in both transcribed and nontranscribed regions, suggesting that mechanisms independent of transcription exist to initiate, maintain, and propagate a common chromatin phenotype over long distances along the kappa chain locus.