Template-engaged and free RNA polymerases during Xenopus erythroid cell maturation

Abstract

Free and template-engaged RNA polymerase activities were measured in nuclei from mature and from different stages of developing Xenopus erythroid cells. Immature erythroid cell nuclei were significantly more active in RNA synthesis than mature erythrocyte nuclei and contain both α-amanitin-sensitive (RNA polymerase II) and -insensitive (RNA polymerase I and III) activities. The transcriptionally inactive mature erythrocyte nuclei retain only RNA polymerase II activity predominantly present as engaged transcription complexes but with a restricted ability to elongate RNA chains in vitro. The anionic detergent Sarkosyl greatly stimulates this activity in the same concentration range required to solubilize the majority of chromatin-bound proteins (principally histones) from these highly condensed nuclei. Quantitation of the relative numbers of RNA chains synthesized in the presence and absence of Sarkosyl reveals that this stimulation results from an increase in the average RNA chain elongation rate and not from the activation of a silent RNA polymerase population. The significance of the retention of RNA polymerase II transcription complexes in the transcriptionally inactive, mature erythrocyte nuclei is discussed.