Characteristics of transcriptionally active and inactive neuronal and nonastrocytic glial rat brain chromatin fractions.

Rapid and reliable fractionation of neuronal and nonastrocytic glial (NAG) cerebral rat brain chromatin in transcribable and repressed portions was achieved employing the DNAase II/Mg++-solubility method of Gottesfeld et al. (1974). Compositional and transcriptional properties of these fractions have been investigated. Compared to transcriptionally repressed fractions, template-active neuronal and NAG chromatin fractions are associated with an increased content of nonhistone chromosomal (NHC-) proteins. Both of the transcribable as well as both of the repressed fractions are strikingly different in their composition as assessed by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis. Comparative acid urea gel electrophoretic patterns of histones revealed that histone fraction H 1 is almost completely absent in actively transcribed neuronal chromatin and reduced in the corresponding NAG fraction while in template-inactive neuronal and NAG chromatin all five main histone fractions are present in equal amounts. The total number of RNA initiation sites available for exogenously added homologous RNA polymerase on template-active and -inactive neuronal and NAG chromatin was quantitatively measured under assay conditions completely eliminating reinitiation. Unlike the template-active neuronal and NAG fractions which are differently enriched in RNA initiation sites, transcriptionally more repressed neuronal and NAG fractions demonstrated a minimal ability to initiate RNA synthesis. Under assay conditions allowing repeated initiation of RNA chains at the same initiation site, rat brain RNA polymerase molecules were found to utilize neuronal initiation sites more frequently than NAG ones.