An electron microscopic analysis of transcription of nucleolar chromatin isolated from Tetrahymena pyriformis.

Transcriptionally active nucleoli and solubilized nucleolar chromatin were visualized by electron microscopy. The palindromic structure of the chromatin was demonstrated by spreading the chromatin on glow-discharged grids. In the presence of single-strand binding EcoHDP protein the preribosomal RNA transcripts are seen attached to the RNA-polymerase molecules in the electron micrographs. Each palindrome contains two preribosomal RNA genes. THe strict termination properties of the transcription are indicated by the absence of transcriptional complexes in the distal parts of the molecules. - Investigation of the process of transcription in nucleolar chromatin being depleted of a termination protein by ammonium sulfate-treatment showed in agreement with biochemical studies that only some of the RNA polymerases terminate properly while other transcribe into the distal spacer region. The elongation rate is estimated to be slightly lower than in the gene region. The results are discussed in relation to biochemical studies of the transcriptional properties of the chromatin.